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Final Answers
© 2000-2017   Gérard P. Michon, Ph.D.

Cosmology 101

[...]  assuredly the world was made,
not in time, but simultaneously with time
The City of God  by  Saint Augustin  (354-430) 

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Powers of 10   |   Deep Space Timeline   |   HubbleSite
Monsignor Georges LemaîtreCosmic egg, primitive atom or... Big Bang.
Ned Wright's Cosmology Tutorial by Dr. Edward L. Wright (UCLA).  |  FAQ
Cosmology, Inflation & the Physics of Nothing  by  William H. Kinney  |  CMB
Distances, Volumes, Time Scales in Cosmology by Hans R. de Ruiter.
121 FAQs about Big Bang Cosmology  by Dr. Sten Odenwald
Center for Astrophysics  (CfA)   |   Andrew Hamilton's Homepage
Open Directory > Cosmology  |  Basic Topics in Cosmology (Cornell)
Cosmological Models   |   Alan Guth on The Paula Gordon Show (Audio)
Weighing the Universe (MAP exhibit)   |   End of the Universe (TIME article)
1965: CMB discovered by Arno Penzias and Robert Wilson (Nobel 1978).
First WMAP data   |   A Designer Universe? by Steven Weinberg (Nobel 1979).
High Redshift Supernova Search by Saul Perlmutter at LBL.
The Anthropic Coincidences:  A Natural Explanation  by  Victor J. Stenger
Cosmology Comes of Age  by  Gordon Fraser   (CERN Courier, 1998)
Big Bang Theories  by  Dipen Bhattacharya   (June 2001)
Baryon Acoustic Oscillations  by  Matt O'Dowd   (2016-02-24)
Humor :   A Ridiculously Short History of Time by Eric Oehler
The History of the Universe in 200 Words or Less by Eric Schulman

DMOZ: Cosmology

Videos :  New Dimension Media #22 :   Overview of the Universe  (44.1 MB)
NASA TV  live coverage  of Shuttle missions, etc.
Origin of the Universe  by Stephen Hawking  1  |  2  |  3  |  4  |  5
Cosmology at YearlyKos 2007, by Sean M. Carroll (CalTech)   1  |  2
Are we real?  (Martin Rees)   1  |  2  |  3  |  4  |  5
Cosmology  (Leonard Susskind)   1 | 2 | 3 | 4 | 5 | 6 | 7 | 8
Aspects of Eternal Inflation  by Lenny Susskind  (IAS, July 2011)  1 | 2 | 3 | 4
Triumphs and Challenges for Modern Cosmology  by  James E. Peebles
Existence of Black Holes  |  Journey into a Black Hole   by Tony Darnell
The Cosmic Web (1:06:13)  Q&A (18:33)  by  J. Richard Gott  (RI, 2017-07-12).
Project Cassiopeia :
Faster than Light:  Universe Created out of Nothing  |  Expanding Universe
In the background of this page is the Hubble Ultra Deep Field (2003-2004).  Courtesy of NASA.
See Video:  The Hubble Deep Field: The Most Important Image Ever Taken by Tony Darnell

 International Year 
 of Astronomy (2009)


(2009-09-21)   Olbers' Paradox
In an infinite and unchanging Universe, the night sky would be white!

Since the night sky isn't brightly lit, the obvious logical conclusion is that the Universe is  not  infinite and unchanging.  We now know that it's finite and expanding.  Historically, the argument presented by Olbers helped clarify that viewpoint.  Arguably, the night sky is neither bright nor dark, it's just darker than it used to and it's dimming steadily...

Nowadays, Heinrich Olbers (1758-1840) is often presented as an amateur mathematician because he earned a living as a physician.  However, he must be credited with several important contributions to astronomy, including an efficient method for computing cometary orbits (1779) and the discovery of two of the three largest asteroids,  2 Pallas (28 March 1802)  and  4 Vesta (29 March 1807).

The largest and most massive asteroid is  1 Ceres,  which was discovered on the first day of the nineteenth century  (1 January 1801)  and is now classified as a dwarf planet.

Olbers' Paradox  did not originate with Olbers but he used his considerable reputation to popularize the argument.  In the cosmological view where outer space consists only of stars and empty space, ... / ...

 Come back later, we're
 still working on this one...

Olbers' Paradox (1823)   |   Heinrich Olbers (1768-1840)

 The Milky Way  (2009-09-21)   Galaxies
A short history of a mind-boggling idea.

The Milky Way  (latin: Via Lactea)  is the dim stripe of nebulous light which can be seen on a clear night in the direction of Sagittarius.  Telescopes resolve that cloud into a system of many stars to which our own Sun actually belongs.  It was a tremendous leap to imagine that all the fuzzy  spiral nebulae  which can be observed through telescopes are giant systems of stars similar to our own but at very great distances.

 Pierre Mechain
 (1744-1804)  The famous catalog of 110 nebulae  (including  spiral nebulae)  which  Charles Messier (1730-1817) and Pierre Méchain (1744-1804) published in 1777 was arguably little more than a list of  nuisances  outside of the  solar system  which might hinder the hunt for glamorized  comets !

 Pierre-Simon Laplace
 (1749-1827)  Pierre-Simon Laplace (1749-1827) advocated the dominant (misguided) opinion that the  spiral nebulae  were rotating clouds forming new stars, according to the nebular hypothesis which had been formulated in 1734 by Emanuel Swedenborg (1688-1772) to explain the formation of the Sun itself.

 Emanuel Swedenborg 
 (1688-1772)  Curiously, Swedenborg had also put forth another  unrelated  seminal idea which would ultimately lead to the correct understanding of the true nature of the spiral nebulae  (and the downfall of Laplace's views).

Indeed, Swedenborg envisioned a definite order among visible stars, within a huge  local  "starry sphere".  Although the details of his description are ambiguous and incompatible with modern views  (it's unclear whether he thought of the  Via Lactea  as a polar axis or an equatorial ring for that sphere).  Swedenborg broke fantastic new grounds when he suggested the mind-boggling possibility that there could be many other such "starry spheres" at very large distances...

Around 1731, similar speculations were being made independently by the gifted son of a well-to-do carpenter and land owner from the hamlet of  Byers Green (6 miles to the south of Durham City)...  In 1750, having fulfilled his unlikely destiny of becoming an astronomer, Thomas Wright (1711-1786)  published his proposal that the distant starry formations envisioned by Swedenborg  (henceforth called  galaxies )  might actually be visible to us in the form of  nebulae.  Wright also explained the appearance of the  Milky Way  as "an optical effect due to our immersion in what locally approximates to a flat layer of stars."

As he wrote that, Wright was still thinking of this "layer" as part of a large hollow spherical distribution  (actually, it's just a finite flat disk).

Both ideas were enthusiastically embraced by the philosopher Immanuel Kant (1724-1804) who credited Wright (but not Swedenborg) for them, although he candidly admitted (in print) that he had never read Wright's book !  Kant coined the popular term of  Island Universes,  which he (correctly) envisioned as fairly thin rotating disks of very many stars  (thus disgarding, possibly involuntarily, Wright's misconceptions about hollow spherical distributions of stars).  In 1755, Kant had this to say in  Universal Natural History and the Theory of Heavens :


 Come back later, we're
 still working on this one...

In 1910, Vesto Melvin Slipher (1875-1969) entered the scene to argue Laplace's case proving that, against all expectations, the  other  was correct!

The question concerned the nature of the so-called  nebulae  (in particular  spiral nebulae)  which were puzzling astoonomers.  Unlike comets, they were clearly outside the Solar system; yet their distances did not make them look pointlike, like stars. 

 Come back later, we're
 still working on this one...

History of our Understanding of a Spiral Galaxy  (M33)  by Kurtiss J. Gordon  (1969)

(2002-07-30)   The Cosmological Principle

The  Cosmological Principle  says that, when viewed on a large enough scale, our physical Universe is essentially homogeneous and isotropic.

In other words, the distant Universe looks roughly the same in any broad direction from any typical point (technically, such "typical" points are comoving points in free fall).  Thus, the reason why the Earth cannot be at the "center" of the Universe, as once thought, is that there is no such "center" (alternately, the "center" is anywhere).  In that respect, the Universe resembles the surface of a perfect sphere:  All points are equivalent and no direction is special.

The idea that the Earth is not at the "center" of the Universe is  ancient,  but it was suppressed for a long time and its current prevalence is fairly recent...

In The Sand Reckoner (c.213 BC), Archimedes of Syracuse (287 BC-212 BC) reports that, according to Aristarchus of Samos (310 BC-c.230 BC), the Earth revolves around the Sun.  However, the opposite viewpoint advocated by Aristotle and Ptolemy later became official Church dogma and remained so for centuries.  The heliocentric idea was thus considered a dangerous heretical view when it was revived in 1514, by a Pole named Copernicus (1473-1543).  In spite of the courageous support of Kepler (1571-1630) and Galileo (1564-1642), Church coercion would not allow the "new" perspective to prevail easily (to say the least).  The Italian philosopher Giordano Bruno (1548-1600) was a noted early supporter of the Copernican heliocentric theory:  He was arrested in 1592, underwent a lengthy trial, refused to recant, and was burned at the stake in 1600.  This goes a long way toward explaining why Galileo "chose" to recant when he was similarly charged, in 1633.

However, once this viewpoint is adopted, it's only natural to think that the Earth should not occupy any special place whatsoever in the Universe as a whole.  After the discovery that the  Milky Way  (which harbors our own Solar System)  is only one of many similar galaxies, it seemed natural to assume that all such galaxies are essentially placed on an equal footing.  At a sufficiently large scale, the distant Universe should look essentially the same in any direction from any typical galaxy.

This statement was first called the Cosmological Principle by the British astrophysicist Edward Arthur Milne (1896-1950).  Before the Cosmological Principle was even known by that name, it had been used by the Russian cosmologist  Alexander Aleksandrovich Friedmann  (1888-1925), who should be given credit for the idea of an expanding Universe.

In 1922, Friedmann devised a model of an expanding Universe obeying both the Cosmological Principle and the equations of General Relativity, without the need for the so-called Cosmological Constant L (which Albert Einstein had introduced mostly to accomodate the [then] prevalent idea of a static Universe).  In 1929, the American astronomer Edwin Hubble independently discovered the first observational evidence of the expansion of the Universe...

There is now overwhelming observational evidence for the validity of the Cosmological Principle from careful measurements of the so-called Cosmic Microwave Background (CMB) which was discovered in 1964-65 by Arno A. Penzias and Robert W. Wilson.  The CMB has been found to be isotropic to a precision of about one part in 100 000.

(2007-09-09)   Big Bang :  The vision of  l'abbé Lemaître  (1927)
The term "Big Bang" was coined  in jest  by Fred Hoyle in 1956.

The idea that the entire Universe could have originated from a single pointlike "primeval atom" was first formulated in 1927 by the Belgian mathematician, Abbé  Georges Lemaître (1894-1966) in a momentous article:

  • Un Univers homogène de masse constante et de rayon croissant rendant compte de la vitesse radiale des nébuleuses extragalactiques
    Annales de la Société scientifique de Bruxelles   A47, pp.49-59  (1927)
  • A homogeneous universe of constant mass and increasing radius accounting for the radial velocity of extra-galactic nebulae
    Notices of the Royal Astronomical Society  91, pp.483-490  (1931)

Lemaître had been ordained a catholic priest in 1923.  He then studied General Relativity at Cambridge under  Arthur Eddington  and went on to MIT.  In 1925, he started lecturing at the Université catholique de Louvain  (UCL)  and accepted a full-time position there in 1927, as he was obtaining his Ph.D. from MIT.

Georges Lemaître presented his  hypothesis of the primeval atom  as describing a day without yesterday.  This idea is now universally called the  Big Bang theory.

Curiously, the name "Big Bang" was originally a  derogatory  term coined by  Fred Hoyle  in 1956 to  mock  the concept...  Hoyle was then a leading proponent of a rival theory which is now all but forgotten.

The theory became universally accepted once the theoretical investigations of  Stephen Hawking  and others proved that General Relativity and the observed expansion of the Universe do imply such a pointlike origin.  The viewpoint has even become part of the official Catholic doctrine  (Msgr. Lemaître was made an honorary prelate in 1960, by Pope John XXIII).

Although Georges Lemaître himself was not an official participant in the 1927 Solvay conference in Brussels  (on the foundations of Quantum Mechanics)  he was residing nearby and met with Einstein on that occasion.  Einstein was not impressed at the time; he was quoted as saying:  "Your calculations are correct, but your grasp of physics is abominable."  However, several years later (c.1935) Einstein would reportedly applaud Lemaître's ideas as "the most beautiful and satisfactory explanation of creation to which I have ever listened".

What prompted that enthusiastic reaction from Einstein seems to have been Lemaître's description of  cosmic rays  as possible left-overs from the primeval explosion.  It turns out that this early insight does not really apply to high-energy cosmic rays  (as described by Robert Millikan)  but to the low-energy photons of the  cosmic microwave background, discovered by  Penzias & Wilson  in 1965, a few months before Lemaître passed away...

Big-Bang Nucleosynthesis (BBN)

(2003-07-14)   The Cosmic Microwave Background (CMB)
What's the energy density of the Cosmic Microwave Background today?

The Cosmic Microwave Background is a gas of photons with a  blackbody  spectrum, whose current temperature has been measured to be:

T   =   2.728(2) K

The energy density [energy per unit of volume] contributed by the photons whose frequencies are between n and n+dn is given by Planck's formula:

un dn   =   
8p hn3 dn
c3 ( exp( hn / kT ) - 1 )

Using the variable   x = hn / kT , the energy density of all photons is thus:

un dn   =   
8p k4 T 4 x3 dx
h3 c3 ( e x - 1 )
8p5 k4 T 4
15 h3 c3

In the above, the definite integral of   x3/( e x - 1 ) was obtained as the sum (for n = 1 to ¥) of the definite integrals of  x3 e -nx :  As the n-th term is 6/n4 the whole sum is  p4/15  (the reciprocals of fourth powers add up to p4/90).

The total energy density of  blackbody  radiation is thus proportional to the fourth power of the absolute temperature T.  In terms of the Stefan-Boltzmann constant (s) the above is equal to  (4s/c) T 4.

To a CMB temperature of 2.728(2) K corresponds an energy density of 4.190(13) 10-14 Pa  (1 Pa equals one joule per cubic meter) which is about 260 electronvolts per liter, or 0.26 eV/cc.

Number of Photons in Blackbody Radiation:

Since each has energy hn, the density of the photons is the following integral, whose value involves Apéry's number z(3), the sum of the reciprocal cubes :
  dn     =    
16p z(3) k3 T 3
h3 c3
For the CMB, this is about 410 000 photons per liter (410 photons per cc).

Average Energy of a Thermal Photon:

It's the ratio of the total energy density to the density of photons, namely:
30 z(3)
  kT     =     ( 2.70117803291906389613472623...)   kT

Median Energy of a Thermal Photon:

m kT   »   2.35676305705  kT
(where m is given by the relation at right)          
x2 dx
( e x - 1 )

Thermal Photon at the Peak of the Frequency Spectrum:

When the spectral density of the blackbody energy is plotted against frequency  (as above)  it's proportional to  x3 / (exp(x)-1)  and reaches a maximum when  exp(-x) = 1-x/3.  That's to say that, in the most energetic interval of frequencies, a thermal photon has the following energy:

h n   =   ( 2.82143937212...) kT

Thermal Photon at the Peak of the Wavelength Spectrum:

In the nineteenth century, the spectral energy per wavelength interval  (instead of frequency interval)  was commonly plotted.  The maximum of  that  diagram is reached at a totally different point corresponding to the solution of  exp(-x) = 1-x/5  namely:

h n   =   ( 4.965114231744...) kT

Cosmic Microwave Background Radiation  by  Ed Copeland  (video by Brady Haran, 2013-03-26).

(2002-07-24)   Redshift
How is redshift defined?  What's a cosmological redshift?

When some signal [sound, light, etc.] emitted at a frequency n is observed at frequency  n / (1+z) , the quantity z is called the redshift of the source for the observer.  In the case of visible light, a positive redshift does make the source appear redder (a negative redshift makes it look bluer and the opposite of a redshift is thus sometimes called a blueshift).

Redshift may have a number of combined causes, including the classical Doppler effect (which depends only on the radial velocity of the source), and the time dilation at the source due to its speed (Special Relativity) and/or surrounding gravity (General Relativity).  Finally, for very distant sources, there is also a cosmological redshift due to the fact that the wavelength of a traveling signal increases in the same proportion as the Universe expands.  In other words, light which was emitted from cosmic distances, when the Universe was (1+z) times smaller than now, is currently observed with a cosmic redshift equal to z.

The quantity   1+z   is the ratio of the observed wavelength to the emitted one and may be expressed as a simple product of several factors.  Each of these correspond to one of the  four  causes of  redshift listed above  (some authors quote only  three  such causes by viewing both types of Doppler shifts as a single aspect of the same phenomenon; I beg to differ):

1 + z     =    (1 + v/u)   
Ö 1-b2
Ö 1 - GM / Rc2
Gravitational  Cosmic 
Doppler Effect General Relativity

Rarely, if ever, are the quantified effects of the four causes of redshift explicitly unified in this way.  Almost always, that grand formula is reduced down to only one or two dominant redshift factors.

The symbols have the following meanings:

  • v is the radial speed of the source.  For cosmological distances and/or curved signal propagation, this is defined in terms of the velocities of the observer and the source with respect to local comoving points at rest in the CMB:  The "radial" speed v is actually the difference between the projections of these velocities on the local tangents to the signal's "path". 
  • u is the celerity of the signal [its phase speed].  For light in a vacuum, u = c = 299792458 m/s.  For sound in dry air (20°C), u » 343.37 m/s. 
  • b is the ratio of the speed of the source to c (the speed of light).
    (There may be a nonzero transverse velocity, in which case b > v/c.) 
  • G is the gravitational constant.  M is the mass of some dominant nonrotating spherical body at a distance R from the source.  (The gravitational redshift factor given here as an example would, of course, be different for other mass distributions, but it's usually a good enough approximation whenever there's no rapidly rotating black hole or neutron star in the immediate vicinity...) 
  • The cosmic redshift factor T/To is the ratio of the "old" temperature T of the Cosmic Background at the source to the "newer" value seen by the observer ( To is currently about 2.728 K).  This factor is actually equal to Do / D, where D is any distance characterizing the whole Universe, like the average distance between major galaxies, or the wavelength of a typical background photon (which is indeed inversely proportional to T ).

Astronomers observe the redshift (z) directly by measuring the wavelengths of known lines in the atomic spectra of the light emitted by a distant source.  However, there is a dubious tradition to quote also the apparent recession speed of such distant sources (defined as the purely radial velocity of a nearby source with the same redshift, in the absence of General Relativistic effects).  This is obtained by retaining the first two factors of the above formula, (letting u = c and b = v/c), so that (1+z) 2 is  (1+b)/(1-b)  and we have:

v/c   =   b   =   [(1+z)2 -1] / [(1+z)2 +1]   =   z / [1 + z/2 + z2/(4+2z) ]

For example, a redshift  z = 1  corresponds to exactly 60% of the speed of light, whereas  z = 2  is 80% of the speed of light, and  z = 6  is (exactly) 96% of the speed of light...  (Again, in a cosmological context, it's best to quote only  z  and ignore this dubious "translation".)

In April 2009, a cosmological redshift of  8.2  was observed for the gamma-ray burst identified as GRB 090423 (which lasted for a few seconds, during the violent implosion of a short-lived massive star into a black hole).  At the time, that object was the oldest and most distant ever seen.  The light we saw from it had been emitted when the Universe was  9.2  times smaller than today  (the temperature of the CMB was then about  25 K).

In October 2010, one galaxy with a redshift of  8.55  (HUDF.YD3)  was found among the 10000 galaxies of the Hubble Ultra Deep Field.

The oldest light we can observe is that of the CMB itself.  It was emitted when the Universe became transparent to electromagnetic radiation, at a temperature of about 3000 K.  This corresponds to a redshift of about  1100.

Butchered in Yahoo! Answers (2010-11-18)   Multiple Choice Exam
In Einstein's General Theory of Relativity, the redshift of galaxies in the Universe is correctly interpreted as:
  1.   A Doppler-shift due to the motions of the galaxies through expanding space.
  2.   An "aging" of the light.
  3.   Space itself is expanding with time; the wavelengths of photons are stretched
    while they travel through the expanding space.
  4.   The difference in temperatures of distant and nearby galaxies.

Clueless students who systematically pick the longest answers would enjoy an unfair advantage here:  The only correct answer is (c).

Answer (a) was once used by Edwin Hubble and others  outside  of the framework of General Relativity  (it still appears occasionally in misguided essays).  However, this is not acceptable in the proper context of General Relativity where typical galaxies occupy locations that are as motionless as the expansion of the Universe can possibly allow  (i.e., comoving points with fixed coordinates).  As discussed above, the Doppler effect is  another  cause of redshift which is unrelated to the  cosmological redshift.

Answer (b) conjures up other deprecated viewpoints which are not compatible with  General Relativity,  unless you redefine "aging" of light as the matching of the wavelength of traveling photons to the changing scale of the Universe described by the correct answer (c)

Answer (d) is wrong but barely so.  It would be correct if we could interpret "temperature of a galaxy" as the temperature of the  Cosmic Background  around that galaxy at the time when the light we see was emitted from it.

(2010-12-19)  Hubble Law vs. Doppler Effect (my own answer to Agasthian on "Yahoo! Answers")

(2002-12-09)   Hubble Law & Hubble Flow
What is Hubble's "constant"?

Arguably, modern cosmology originated in 1917 at the Lowell Observatory, when Vesto Melvin Slipher (1875-1969) observed that distant galaxies are all receding from our own Milky Way.  In 1929, Edwin P. Hubble (1889-1953) discovered (from sketchy observational data) that the recession speed (v) of a galaxy is roughly proportional to its distance from us (d).  The nonrelativistic coefficient of proportionality is now called Hubble's constant (H or H):

v   =   H d

Hubble's constant (H) actually describes the rate of expansion of the Universe, and its value evolves as the Universe ages.  Simple models of the Universe make the product of H and the Age of the Universe equal to a dimensionless number that depends on specific assumptions:  This product would be equal to 1 in a Universe of very low density [H would be the reciprocal of the Universe's age], but it would be 2/3 in a flat Universe (W = 1) dominated by ordinary matter, and only 1/2 in a radiation-driven expansion phase (the fireball conditions which prevailed for less than 56 000 years).  On the other hand, if some form of exotic stuff and/or a nonzero cosmological constant dominates the large-scale structure of the Universe (as modern data may indicate), the above product could be equal to or larger than one, so the Universe might be older than 1/H.

The actual value of  H  is difficult to determine experimentally, mostly because it's difficult to determine the distance to an object that's far enough to make its [unknown] proper motion a negligible factor in its observed redshift.  The latest estimates place H somewhere between 68 km/s/Mpc and 75 km/s/Mpc.

The reciprocal of  H  is sometimes called the Hubble time, and the Age of the Universe is commensurate with it.  One "s-Mpc/km" is 977 792 221 400 years, and the Hubble time corresponding to the above values of  H  is thus 75 or 68 times smaller than this, namely between 13 and 14.4 billion years...

Refined Hubble Constant ...   by Adam Riess et al.   H = 74.2(36) km/s/Mpc   (2009-05-07)

(2002-07-24)   W
What is meant by "critical density"?  What's the omega (W) constant?

Following Steven Weinberg (The First Three Minutes, 1977) we'll introduce the notion of critical density in the framework of Newtonian mechanics.  It turns out that the relativistic computation that we'll outline next gives the same final result,  provided  the density "r" is understood to include the density of energy divided by c and a corrective term for what's now called  dark energy.  First things first, here's the simple Newtonian argument:

Consider a sphere of radius R much smaller than the whole universe, but large enough to apply the Cosmological Principle.  If r is the average mass density of the Universe, such a sphere is roughly homegeneous and its total mass M is equal to r times its volume, namely:

M   =   4pR3r/3

The (Newtonian) potential energy of a galaxy of mass m near the surface of the sphere is -mMG/R (where G is Newton's Universal Constant of Gravitation):

- mMG/R   =   - 4pmR2rG/3

On the other hand, this galaxy has a (purely radial) speed V = HR given by Hubble's Law (H = H(t) being the value of Hubble's constant at the present time t) and its kinetic energy is therefore:

½ m V2   =   ½ m H2 R2

The total energy of the galaxy is the sum of the above two terms and remains constant as the Universe expands:

m R2  [ H2/2  -  4prG/3 ]

If this total energy is positive, the galaxy will eventually escape to infinity with some kinetic energy left over.  If it's negative, this won't happen and, in fact, the Universe's expansion will eventually stop and reverse (the Universe will then collapse).  Between these two alternatives is the critical case where the bracket in the above expression is precisely zero, whereby the Universe keeps on expanding forever, but just barely so  (the relative speed of two typical galaxies eventually approaches zero but their distance still keeps growing to infinity).  The above expression shows that this happens precisely when the density r of the Universe is equal to the following quantity ro, which is called the critical density :

ro   =   3 H2 / 8pG               W = r / ro

The ratio (W) of the actual density r to the critical density is the famous omega "constant", which determines the ultimate fate of the Universe:  If W is less than or equal to 1, the Universe will expand forever, otherwise it will eventually collapse.

W  is not really constant but the sign of  W-1  is  (in this model at least).

As advertised, those results remain valid within  General Relativity,  whereby an homogeneous and isotropic universe  (as envisioned by Friedmann in 1922)  is characterized by two parameters;  an increasing  scale factor  a(t)  (which is a length)  and a dimensionless constant K  (the spatial  Gaussian curvature  being  K/a).  The following relation is known as the  first Friedmann equation :

H2   =   (a'/a)2   º   (8pGr + L) / 3  -  K c2 / a2

Solving for  r  when  L=0  and  K=0  gives the aforementioned value of  r.

Our previous Newtonian interpretation remains valid  only  if  Einstein's field equations  hold with a vanishing  cosmological constant  (L=0).  Otherwise, a positive  cosmological constant  can even entail an  accelerating  expansion of the Universe,  which is compatible with the  big picture  that emerged from observational data in 1998.  There's simply no Newtonian explanation for  that !

More precisely, according to the above  Friedmann equation,  a positive cosmological constant  L  can be tallied as a supplemental density  L / 8pG  attributed to  dark energy  (this is just a name)  which is added to the combined total from ordinary matter and energy as well as  dark matter.

Dark matter is distinct from dark energy  (which is uniform in space and repulsive on a cosmological scale)  because it can be lumped locally and features attractive gravitation  (as can be detected by gravitational lensing and/or the rotational speed of neighboring visible matter).  Yet, dark matter is "stuff" which lacks any detectable non-gravitational interaction with ordinary matter or radiation.  It cannot be made of any of the particles which Science has cataloged so far.

If the overall geometry of our universe wasn't very nearly flat  (like a  Friedmann universe  with  K=0)  then it would either have collapsed or thinned out beyond recognition a long time ago.  The current prevalent view is thus that  dark energy  is due to a positive cosmological constant that accounts for whatever is needed to balance the above  Friedmann equation  (assuming vanishing  Gaussian curvature).  This would mean that  empty space  accounts for about  70%  of the total tally  (i.e, space itself and all its matter-energy  content,  including  dark matter).

Mind-boggling, isn't it?

(2003-07-16)   Look-Back Time
How is the "look-back time" of distant objects determined?

The redshift of a very remote object is observed directly.  All other indicators of its distance depend on some  cosmological model  of the Universe.

In particular, the look-back time of a distant source is defined as the time ellapsed since the light reaching us was emitted.  Because of the Universe's expansion, such a distant source is always farther away than what would be naively estimated by multiplying its look-back time by the speed of light.

Well, this "naive" estimate is one possible definition of cosmological distance, which may be called the distance "to us".  The distance "from us"  (which could be defined, in this context, as the time it would take for photons we send now to reach the distant object)  is certainly larger than that in a rapidly expanding Universe, but it's not infinite unless the expansion is  accelerating  out of control...

Our estimate of look-back time depends on which model of the Universe we rely on.  Currently, one popular view is that the total energy content of the Universe is zero (negative gravitational energy balances matter and other forms of positive energy) and that the existence of some sort of exotic "dark" stuff and/or a nonzero Cosmological Constant makes the Universe behave gravitationally, on a large scale, as if it was effectively empty (its expansion does not slow down).  This viewpoint is illustrated by the last two columns of the following table:

Look-Back Times (Millions of Years) for 2 Cosmic Models & 2 Values of  H
( z )
( b )
( W = 1 )
Zero Total Energy
( Effectively, W = 0 )
75 km/s/Mpc 68 km/s/Mpc 75 km/s/Mpc 68 km/s/Mpc
z » 0z z / H
 (1+z) 2 - 1 
(1+z) 2 + 1
 2      [  
 1 -  
1   ]
vinculum vinculum
 1      z
vinculum vinculum
¥100%86919586 1303714379
If the Universe was indeed dominated by ordinary matter, it would be younger than the oldest stars in it !

(2002-12-09)   Distance
What is distance in a cosmological context?

Astronomers estimate distance in many different ways.  It's not at all obvious that all such methods end up measuring the same thing.  In fact, they don't.

In an observational cosmological context, the distance to a distant object is [probably] best defined as the distance its light has traveled before reaching the observer.  This definition would mean distance and look-back time are simply proportional (the coefficient of proportionality being Einstein's constant).  Thus the relation between distance and redshift would depend, as discussed above, on how the Universe has expanded between the emission and the reception of light.

This observed cosmological distance is thus not a simple concept and it's fairly useless in theoretical speculations, where the distance of an object to the [arbitrary] origin is best defined as the value of a space-coordinate when the time-coordinate is the same [we're talking curvilinear coordinates in curved space, here].  In an expanding universe, this latter flavor of distance is greater than the former one.  [The source has "moved away" after emitting its light.]

The straightforward parallax method, based on Euclidean trigonometry, may not be valid for very large distances and/or when strong gravity is present; the three angles of a large physical triangle may not quite add up to 180°.  Although the parallax angles of galaxies are actually far too small to be measured, we may wonder how trigonometry could be used in principle to measure intergalactical distances...  The very concept of distance is worth questioning under at least three types of extreme conditions:

  • Extremely small scales:  The Planck length (1.6´10-35 m) is the characteristic unit of a scale at which physical space itself is thought to lack any kind of smoothness.  Geometry breaks down when we "look" this close.  This is the not-yet-understood domain of quantum gravity. 
  • Extreme curvature:  Around black holes, our Euclidean intuition fails.  It's best to avoid considering the "distance to the center of a black hole", because this distance would turn out to be infinite under most definitions. 
  • Extremely large scales:  As the Universe expands, so does the distance between two objects sufficiently far apart.  The expansion of the Universe may thus introduce a significant delay in the light signals that go from one object to the other.  It becomes important to state precisely what is meant by "distance" in such a context, as discussed above.

(2002-07-24)   Comoving Points & CMB Anisotropy
What are "comoving points" ?

In the Euclidean space of classical geometry, motion is actually considered relative to some immobile framework of fixed points.  This viewpoint is not a practical proposition within our expanding physical Universe considered as a whole.  Instead, the cosmological approach is to introduce reference points whose relative motions are entirely due to the general expansion of space itself, whatever that may be.  By definition, such points are said to be comoving.

The relative motions of galaxies are not entirely due to the expansion of the Universe (nearby galaxies attract each other) and their centers of mass are thus not strictly comoving.  However, descriptions of our expanding Universe will often discard the distinction for the sake of simplifying the presentation.  The centers of fairly large clusters of galaxies could seem to be slightly better embodiments of comoving points, but such attempted refinements are vastly inferior to the better characterization we shall now give...

The most practical viewpoint is to characterize a comoving point as a point which is at rest with respect to the Cosmic Microwave Background (CMB).  The Sun is not comoving (relative to the CMB, its speed is about 370 km/s ).  Neither is the center of mass of our Local Group of galaxies, which moves at about 600 km/s with respect to the CMB (three dozen galaxies are thus not a large enough chunk of matter to estimate the value of our local Hubble flow).

The dipolar anisotropy of the CMB from our local viewpoint  (which indicates that the Earth and the Sun are not comoving)  was first precisely determined in 1977 by the so-called U2 Anisotropy Experiment which was flown aboard the NASA Ames U2 jet aircraft by a UC Berkeley group.  The results were later confirmed from outer space, by the "Cosmic Background Explorer" (COBE) launched on November 18, 1989.  George Smoot masterminded both projects.

The Universe after 379 000 years,
according to WMAP (Feb. 2003)...
Click for larger picture from NASA. Knowing our own speed in the CMB is just the beginning.  The tiny irregularities in the CMB offer a baby picture of the Universe at the age of about 379 000 years, when it first became transparent.  (A COBE picture made headlines in April 1992.)

On June 30, 2001, NASA launched its "Microwave Anisotropy Probe" (MAP) at a cost of $145 000 000.  It is 45 times more sensitive than COBE and its angular resolution is 33 times better.  MAP was renamed in honor of David T. Wilkinson, who died on September 2, 2002 (WMAP = "Wilkinson Microwave Anisotropy Probe").  It arrived at L2 on Oct. 1, 2001 (the second Lagrange point "L2" is a semi-stable orbital position on the Earth-Sun line, 1.5 million km further from the Sun than the Earth).  A first sky scan was completed in April 2002 and the WMAP results were finally released on February 11, 2003...

George Smoot and John C. Mather (of NASA) have been jointly awarded the 2006 Nobel Prize in physics  "for their discovery of the black-body form and anisotropy of the cosmic microwave background radiation".

(2002-07-30)   The Anthropic Principle

The Anthropic Principle is simply the statement that the Universe we observe must allow intelligent life to evolve, or else we would not be here to observe it.

In any universe with features that rule out intelligent life, there would not be anybody around to wonder why such features exist...  Yet, there is a general feeling that the Anthropic Principle by itself provides a poor sort of explanation.  Indeed, if we were to assume that there's only one possible universe, it seems that there should always be a reason for what we observe, other than our own existence.  Thus, cosmologists often find the Anthropic Principle somewhat repugnant and will invoke it only as a last resort...

The alternative, however, is that there could very well "be" (in some obscure sense) many universes.  Some have intelligent observers in them and some don't.  The Anthropic Principle simply states that our own Universe can only be of the former type.  In fact, André Linde's "chaotic inflation" theories do predict that the creation of a universe like ours is best explained as part of a process which creates a large multiplicity of universes in which the fundamental constants of nature may have different values.  If that viewpoint is correct, there would not be any ultimate explanation for the values of the fundamental physical constants, except that their range should be compatible with the Anthropic Principle...

Now, the tricky part is that the dubious "existence" of other universes is entirely irrelevant, by definition, to the physics of our own Universe.  As an irrelevant assumption does not change anything, we may conclude that the Anthropic Principle (which may or may not be ultimately needed ) is fully justified even if we leave open the "existence" of anything outside of our own Universe.

(2002-10-28)   Dark Matter & Dark Energy

I would like to learn that Newton's laws must be modified in order to correctly describe gravitational interactions at large distances.  That's more appealing than a universe filled with a new kind of particle.
Vera Rubin  (1928-)

Dark Matter & the Pull of Galaxies:

The Sun and other stars have an orbital speed around the Galaxy which is much larger than what it would be if gravitational forces were only due to all the  ordinary  matter we can tally (stars and interstellar gas).  The same observation can be made in other galaxies as well.  Galaxies have massive dark halos which consist of some strange stuff, called dark matter.

Although the early evidence for the existence of dark matter came from galactic rotation curves, the relative speeds of galaxies in some clusters also imply the existence of intergalactic dark matter to hold clusters together (at the large speeds observed, the galaxies would otherwise have flown apart a long time ago).  More localized evidence has also been found recently.

Although most  ordinary  matter actually resides outside of galaxies, all the evidence that has been gathered since 1998 seems to indicate that there's about five times more dark matter than ordinary matter in the Universe.

Dark Energy & the Accelerating Expansion of the Universe:

In 1998, the study of distant Type-1A supernovae  (Nobel 2011)  indicated that the expansion of the Universe is  accelerating.  Such an acceleration can be accounted for by a small positive cosmological constant  (L)  in Einstein's field equations.

This would mean that  empty space  itself has a nonzero energy density of about  0.5 nPa  or  3 keV/cc  (that's roughly equivalent to the rest energy of one helium atom per cubic meter of  empty  space).

This type of energy is called  dark energy  and it accounts for most  (70%)  of the following composition of our Universe  [courtesy of NASA]:

 Composition of the cosmos 
 (Courtesy of NASA)
On the trail of dark energy  by Eric Linder  (CERN Courier, 2003-09-04)
Perimeter Institute Video:  The Dark Matter Mystery:  Stars Are Moving Too Fast
Dark Matter / Dark Energy (The History Channel)...   Dark Matter:  1 | 2 | 3 |   Dark Energy:  4 | 5 |
Space Fan News #77 (by Tony Darnell) :   Dark Energy is Real.  Dark Matter Shows Itself (2012-09-14)
Dark Energy is Real (2012-09-12)   Re:  Tommaso Giannantonio, Robert Crittenden & al. (2012-09-10)
Finding Dark Matter (2012-09-04)   Re:  Detecting the Galactic Haze with Planck (2012-08-27)

Ryan Landfield  (2009-04-10; UA931)   The Pioneer Anomaly
A residual (decaying) sunward acceleration of   8.74(45) 10 -10 m/s2

This is commensurate with the  Hubble acceleration  (i.e., the product of the  Hubble constant  H  by the  speed of light  c).

H c   =   6.9 10 -10 m/s2 

In July 2012,  Slava Turyshev et al.  apparently nailed that coffin with a full quantitative explanation of the "anomalous" acceleration in term of the recoil of infrared photons emitted by the onboard nuclear power and bouncing off the back of the parabolic communication antenna.  This would explain not only the magnitude of the observed effect but also its exponential decrease with time at a rate similar to the  decay  of the plutonium which powers the Pioneer probe  (behind its own antenna).

The Pioneer Probes Are Way Off-Course (5:37)  by  Hank Green  (SciShow Space, 2016-09-27).
Wikipedia :   Pioneer anomaly

Fermi Gamma-ray Space Telescope
HSF - International Space Station
Space Telescope Science Institute | HST
HubbleSite | News & Views
The Hubble Legacy Archive (HLA)
The Hipparcos Space Astrometry Mission
Hipparcos Pinpoints the Stars
Jovian Moon Amalthea (2002-12-10)
A Star with two North Poles (2003-04-22)
Sounds of Pulsars  (Jodrell Bank Pulsar Group)
Dark Energy Found in the Young Universe

Space Headlines

 World Science Underground Ocean on Titan   (2008-03-20)
The Big Bounce   (2006-05-14)
One Universe or Many?   (2006-04-29)
Wall of Galaxies...   (2006-04-29)
Space Rock Risk...   (2006-03-21)

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