Frequently Asked Questions in Cosmology

Tutorial : Part 1 | Part 2 | Part 3 | Part 4 | Age | Distances | Bibliography | Relativity

What is
the currently most accepted model for the Universe?

What is
the evidence for the Big Bang?

What happened
during the Big Bang?

What is
this "anti-gravity"? [The cosmological constant]

Why do
we think that the expansion of the Universe is accelerating?

What
is quintessence?

How old is the
Universe?

If the
Universe is only 14 billion years old, why isn't the most distant object we
can see 7 billion light years away?

If the
Universe is only 14 billion years old, how can we see objects that are now 47
billion light years away?

Is the
Universe really infinite or just really big?

How can the
Universe be infinite if it was all concentrated into a point at the Big
Bang?

How can
the oldest stars in the Universe be older than the Universe?

Can
objects move away from us faster than the speed of light?

What is
the redshift?

Are
quasars really at the large distances indicated by their redshifts?

What
about objects with discordant redshifts, like Stephan's Quintet?

Has the
time dilation of distant source light curves predicted by the Big Bang been
observed?

Are
galaxies really moving away from us or is space just expanding?

Why
doesn't the Solar System expand if the whole Universe is expanding?

Is the
Universe expanding or is it just that our definitions of length and time are
changing?

Why
haven't the CMBR photons outrun the galaxies in the Big Bang?

Where was the
center of the Big Bang?

What
is meant by a flat Universe?

Is the
Big Bang a Black Hole?

What is
the Universe expanding into?

What
came before the Big Bang?

Doug Scott's
Cosmic Microwave Background Radiation (CMBR) FAQ

Can the CMBR
be redshifted starlight?

Why is
the sky dark at night?

Will the
Universe expand forever or recollapse?

Does
entropy prevent a Big Crunch?

What
about the oscillating Universe?

What is
the dark matter?

What
about MOND?

What is
the value of the Hubble constant?

What
can a layperson do in cosmology?

When
will the next WMAP data release occur?

Ask your own
question!

What is the currently most accepted model for the Universe?

The current best fit model is a flat ΛCDM
Big Bang model where the expansion of the Universe is accelerating,
and the age of the
Universe is 13.7 billion years.

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What is the evidence for the Big Bang?

The evidence for the Big Bang comes from many pieces of observational data
that are consistent with the Big Bang. None of these prove the Big
Bang, since scientific theories are not proven. Many of these facts are
consistent with the Big Bang and some other cosmological models, but taken
together these observations show that the Big Bang is the best current model for
the Universe. These observations include:

The darkness of the night sky - Olbers' paradox.

The Hubble
Law - the linear distance vs redshift law. The
data are now very good.

Homogeneity - fair data showing that our location in the Universe is not
special.

Isotropy - very strong data showing that the sky looks the same in all
directions to 1 part in 100,000.

Time
dilation in supernova light curves.

The observations listed above
are consistent with the Big Bang or with the Steady State model, but many
observations support the Big Bang over the Steady State:

Radio source and quasar counts vs. flux. These show that the Universe has
evolved.

Existence of the blackbody CMB. This shows
that the Universe has evolved from a dense, isothermal state.

Variation of TCMB
with redshift. This is a direct observation of the evolution of the
Universe.

Deuterium, 3He, 4He, and 7Li abundances.
These light isotopes are all well fit by predicted reactions
occurring in the First Three
Minutes.

Finally, the angular power spectrum of
the CMB anisotropy that does exist at the several parts per million level is
consistent with a dark matter dominated Big Bang model that went through the
inflationary scenario.

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Why do we think that the expansion of the Universe is accelerating?

The evidence for an accelerating expansion comes from observations of the brightness of
distant supernovae. We observe the redshift of a supernova
which tells us by what the factor the Universe has expanded since the supernova
exploded. This factor is (1+z), where z is the redshift. But
in order to determine the expected brightness of the supernova, we need to know
its distance
now. If the expansion of the Universe is accelerating due to a cosmological
constant, then the expansion was slower in the past, and thus the time
required to expand by a given factor is longer, and the distance NOW is larger.
But if the expansion is decelerating, it was faster in the past and the distance
NOW is smaller. Thus for an accelerating expansion the supernovae at high
redshifts will appear to be fainter than they would for a decelerating expansion
because their current distances are larger. Note that these distances are all
proportional to the age of the Universe [or 1/Ho],
but this dependence cancels out when the brightness of a nearby supernova at
z close to 0.1 is compared to a distant supernova with z close
to 1.

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What is quintessence?

Quintessence, or the fifth essence, is a fifth element beyond the standard
earth, air, fire and water of ancient chemistry. Steinhardt and colleagues have
adopted quintessence as the name for a particular model for the vacuum energy
which causes the accelerating expansion of the Universe. A search of astro-ph on
the LANL preprint server arXiv for
"quintessence" in the abstract hits over 600 articles of which the oldest dates from 1998.

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If the Universe is only 14 billion years old, why isn't the most distant
object we can see 7 billion light years away?

This question makes some hidden assumptions about space and time which are
not consistent with all definitions of distance and time. One assumes that all
the galaxies left from a single point at the Big Bang, and the most distant one
traveled away from us for half the age of the Universe at almost the speed of
light, and then emitted light which came back to us at the speed of light. By
assuming constant velocities, we must ignore gravity, so this would only happen
in a nearly empty Universe. In the empty Universe, one of the many possible
definitions of distance does agree with the assumptions in this question: the
angular size distance, and it does reach a maximum value of the speed
of light times one half the age of the Universe. See Part 2 of the
cosmology tutorial for a discussion of the other kinds of distances which go to
infinity in the empty Universe model since this gives an unbounded Universe.

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If the Universe is only 14 billion years old, how can we see objects that
are now 47 billion light years away?

When talking about the distance of a moving object, we mean the spatial
separation NOW, with the positions of both objects specified at the current
time. In an expanding Universe this distance NOW is larger than the speed of
light times the light travel time due to the increase of separations between
objects as the Universe expands. This is not due to any change in the units of
space and time, but just caused by things being farther apart now than they used
to be.

What is the distance NOW to the most distant thing we can see? Let's take the
age of the Universe to be 14 billion years. In that time light travels 14
billion light years, and some people stop here. But the distance has grown since
the light traveled. The average time when the light was traveling was 7 billion
years ago. For the critical density
case, the scale factor for the Universe goes like the 2/3 power of the time
since the Big Bang, so the Universe has grown by a factor of 22/3 =
1.59 since the midpoint of the light's trip. But the size of the Universe
changes continuously, so we should divide the light's trip into short intervals.
First take two intervals: 7 billion years at an average time 10.5 billion years
after the Big Bang, which gives 7 billion light years that have grown by a
factor of 1/(0.75)2/3 = 1.21, plus another 7 billion light years at
an average time 3.5 billion years after the Big Bang, which has grown by a
factor of 42/3 = 2.52. Thus with 1 interval we got 1.59*14 = 22.3
billion light years, while with two intervals we get 7*(1.21+2.52) = 26.1
billion light years. With 8192 intervals we get 41 billion light years. In the
limit of very many time intervals we get 42 billion light years. With calculus
this whole paragraph reduces to this.

Another way of seeing this is to consider a photon and a galaxy 42 billion
light years away from us now, 14 billion years after the Big Bang. The distance
of this photon satisfies D = 3ct. If we wait for 0.1 billion years, the Universe
will grow by a factor of (14.1/14)2/3 = 1.0048, so the galaxy will be
1.0048*42 = 42.2 billion light years away. But the light will have traveled 0.1
billion light years further than the galaxy because it moves at the speed of
light relative to the matter in its vicinity and will thus be at D = 42.3
billion light years, so D = 3ct is still satisfied.

If the Universe does not have the critical density then the distance is
different, and for the low densities that are more likely the distance NOW to
the most distant object we can see is bigger than 3 times the speed of light
times the age of the Universe. The current best fit model which has an
accelerating expansion gives a maximum distance we can see of 47 billion light
years.

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Is the Universe really infinite or just really big?

We have observations that say that the radius of curvature of the Universe is
bigger than 70 billion light years. But the observations allow for either a
positive or negative curvature, and this range includes the flat Universe with
infinite radius of curvature. The negatively curved space is also infinite in
volume even though it is curved. So we know empirically that the volume of the
Universe is more than 20 times bigger than volume of the observable Universe.
Since we can only look at small piece of an object that has a large radius of
curvature, it looks flat. The simplest mathematical model for computing the
observed properties of the Universe is then flat Euclidean space. This model is
infinite, but what we know about the Universe is that it is really big.

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How can the oldest stars in the Universe be older than the Universe?

Of course the Universe has to be older than the oldest stars in it. So this
question basically asks: which estimate is wrong -

The age of the Universe

The age of the oldest stars

Both

The age of the Universe is determined from its expansion
rate: the Hubble
constant, which is the ratio of the radial velocity of a distant galaxy to
its distance. The radial
velocity is easy to measure, but the distances are not.
Thus there is currently a 11% uncertainty in the value of the
Hubble constant measured directly by the Hubble Space Telescope. John Huchra
gives a good discussion
of the historical uncertainties in the Hubble constant since even before
Hubble's work. There is now a more precise but more indirect determination from
WMAP observations of the CMB anisotropy.

The estimated age of the Universe has been increased by the observations of
an accelerated expansion of the Universe. The current best value is 13.7 +/-
0.2 billion years from WMAP.

Determining the age
of the oldest stars requires a knowledge of their luminosity, which depends on
their distance. This leads to a 15% uncertainty in the ages of the oldest stars
due to the difficulty in determining distances.

Thus the discrepancy between the age of the oldest things in the Universe and
the age inferred from the expansion rate was always within the margin of error.
In fact, in 1997 improved distances from the HIPPARCOS satellite suggested that
the oldest stars were younger, and the WMAP results in 2003 suggest that the
Universe is older, so the discrepancy has disappeared.

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Can objects move away from us faster than the speed of light?

Again, this is a question that depends on which of the many distance
definitions one uses. However, if we assume that the distance of an object
at time t is the distance from our position at time t to the
object's position at time t measured by a set of observers moving with
the expansion of the Universe, and all making their observations when they see
the Universe as having age t, then the velocity (change in D
per change in t) can definitely be larger than the speed of light. This
is not a contradiction of special relativity because
this distance is not the same as the spatial distance used in SR, and the age of
the Universe is not the same as the time used in SR. In the special case of the
empty Universe, where one can show the model in both special relativistic
and cosmological coordinates, the velocity defined by change in cosmological
distance per unit cosmic time is given by v = c ln(1+z), where
z is the redshift,
which clearly goes to infinity as the redshift goes to infinity, and is
larger than c for z > 1.718. For the critical density Universe, this
velocity is given by v = 2c[1-(1+z)-0.5] which is larger
than c for z > 3 .
For the concordance model based on CMB data
and the acceleration
of the expansion measured using supernovae, a flat
Universe with OmegaM
= 0.27, the velocity is greater than c for z > 1.407.

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What is the redshift?

The redshift of
an object is the amount by which the spectral lines in the source are shifted to
the red. That is, the wavelengths get longer. To be precise, the redshift is
given by
z = [λobs-λem]/λem

where λem is the emitted wavelength of a line, which is known
from laboratory measurements, and λobs is the observed wavelength of
the line. In an expanding Universe, distant objects are redshifted, with z =
Ho D/c for small distances. This law was discovered by Hubble
and Ho is known as the Hubble constant.

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Are quasars really at the large distances indicated by their redshifts?

The short answer is

Yes!

Stockton
(1978, ApJ, 223, 747) observed faint galaxies near in the sky to bright
quasars at moderate redshifts. He chose quasars with moderate redshifts so he
would still be able to see galaxies at the redshift of the quasar. He found that
a good fraction of the redshifts of the faint galaxies agreed with the redshifts
of the quasars. In other words, quasars are associated with galaxies that have
the same redshift as the quasar and have just the brightness expected if the
quasars are at their cosmological distances. Thus at least some quasars are at
the distance indicated by their redshifts, and this includes some of the most
luminous quasars: for example 3C273. Thus the simple
answer selected by Occam's razor is that all quasars are at the distances
indicated by their redshifts.

A further argument in favor of cosmological redshifts for quasars is the
essentially perfect rank ordering implied by the fact that quasar absorption
line system always have redshifts less than or equal to the quasar emission line
redshift. In gravitational lens systems, the redshift of the lens is always less
than the redshift of the lensed object. Thus intervening systems like lensing
galaxies or absorbing clouds, which obviously have smaller distances than the
quasars, also have smaller redshifts.

The statistical arguments advanced by Arp and others in favor of anomalous
quasar redshifts are often incorrect.

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What about objects with discordant redshifts, like Stephan's Quintet?

One famous example of objects with different redshifts appearing in the same
part of the sky is Stephan's Quintet. But
the low redshift galaxy (in the lower left) is obviously more resolved into
stars and looks "bumpier". By the surface brightness fluctuation method of distance
determination, this bumpiness means that the low redshift galaxy is indeed much
closer to us than the other four members of the quintet.

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Has the time dilation of distant source light curves predicted by the Big
Bang been observed?

This time dilation is a consequence of the standard interpretation of the
redshift: a supernova that takes 20 days to decay will appear to take 40 days to
decay when observed at redshift z=1. The time dilation has been observed, with 5
different published measurements of this effect in supernova light curves. These
papers are:

Leibundgut etal,
1996, ApJL, 466, L21-L24

Goldhaber etal, in
Thermonuclear Supernovae (NATO ASI), eds. R. Canal, P. Ruiz-LaPuente, and J.
Isern.

Riess etal, 1997,
AJ, 114, 722.

Perlmutter etal,
1998, Nature, 391, 51.

Goldhaber etal, ApJ in
press.

These observations contradict tired light models of
the redshift.

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Are galaxies really moving away from us or is space just expanding?

This depends on how you measure things, or your choice of coordinates. In one
view, the spatial positions of galaxies are changing, and this causes the
redshift. In another view, the galaxies are at fixed coordinates, but the
distance between fixed points increases with time, and this causes the redshift.
General relativity explains how to transform from one view to the other, and the
observable effects like the redshift are the same in both views. Part 3 of the
tutorial shows space-time diagrams for the Universe drawn in both ways.

In the absence of the cosmological constant, an object released at rest with
respect to us does not then fly away from us to join the Hubble flow. Instead,
it falls toward us, and then joins the Hubble flow on the other side of the sky,
as discussed by Davis,
Lineweaver & Webb (2003, AJP, 71, 358). In what are arguably the most
reasonable coordinates, the cosmic time t and the distance
D(t) measured entirely at the cosmic time t, the acceleration
is given by g = -GM(r<D)/D2 where M(r<D) is
the mass contained within radius D. This gives g =
-(4*pi/3)*G*(rho(t)+3P(t)/c2)*D(t). The
3P/c2 term is a general relativistic correction to the
otherwise Newtonian dynamics. Galaxies all move under the influence of this
acceleration and their initial position and velocity. In other words, F =
ma and gravity provides the force. Nothing extra or weird is needed.

Also see the Relativity
FAQ answer to this question.

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Why doesn't the Solar System expand if the whole Universe is expanding?

This question is best answered in the coordinate system where the galaxies
change their positions. The galaxies are receding from us because they started
out receding from us, and the force of gravity just causes an acceleration that
causes them to slow down, or speed up in the case of an accelerating expansion.
Planets are going around the Sun in fixed size orbits because they are bound to
the Sun. Everything is just moving under the influence of Newton's laws (with
very slight modifications due to relativity). [Illustration] For the
technically minded, Cooperstock et al.
computes that the influence of the cosmological expansion on the Earth's orbit
around the Sun amounts to a growth by only one part in a septillion over the age
of the Solar System. This effect is caused by the cosmological background
density within the Solar System going down as the Universe expands, which may or
may not happen depending on the nature of the dark matter. The mass loss of the
Sun due to its luminosity and the Solar wind leads to a much larger [but still
tiny] growth of the Earth's orbit which has nothing to do with the expansion of
the Universe. Even on the much larger (million light year) scale of clusters of
galaxies, the effect of the expansion of the Universe is 10 million times
smaller than the gravitational binding of the cluster.

Also see the Relativity
FAQ answer to this question.

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Is the Universe expanding or is it just that our definitions of length and
time are changing?

The definitions of length and time are not changing in the standard model.
The second is still 9192631770 cycles of a Cesium atomic clock and the meter is
still the distance light travels in 9192631770/299792458 cycles of a Cesium
atomic clock.

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What is meant by a flat Universe?

The Universe appears to be homogeneous and isotropic, and there are only
three possible geometries that are homogeneous and isotropic as shown in Part 3. A flat space
has Euclidean
geometry, where the sum of the angles in a triangle is 180o. A curved
space has non-Euclidean
geometry. In a positively curved, or hyperspherical space, the sum of the
angles in a triangle is bigger than 180o, and this angle excess gives
the area of the triangle divided by the square of the radius of the surface. In
a negatively curved or hyperbolic space, the sum of the angles in a triangle is
less than 180o. When Gauss
invented
this non-Euclidean geometry he actually tried measuring a large triangle, but he
got an angle sum of 180o because the radius of the Universe is very
large (if not infinite) so the angle excess or deficit has to be tiny for any
triangle we can measure. If the radius is infinite, then the Universe is flat.

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Bolyai
developed this geometry and published it, whereupon Gauss wrote to Bolyai's
father: "To praise it would amount to praising myself. For the entire content of
the work ... coincides almost exactly with my own meditations which have
occupied my mind for the past thirty or thirty-five years." And Lobachevsky
had published very similar work in the obscure Kazan Messenger.

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Is the Big Bang a Black Hole?

The Big Bang is really nothing like a black hole. The Big Bang is a
singularity extending through all space at a single instant, while a black hole
is a singularity extending through all time at a single point. For more, see the
sci.physics FAQ.

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What is the Universe expanding into?

This question is based on the ever popular misconception that the Universe is
some curved object embedded in a higher dimensional space, and that the Universe
is expanding into this space. This misconception is probably fostered by the balloon analogy which
shows a 2-D spherical model of the Universe expanding in a 3-D space. While it
is possible to think of the Universe this way, it is not necessary, and
there is nothing whatsoever that we have measured or can measure that will show
us anything about the larger space. Everything that we measure is within
the Universe, and we see no edge or boundary or center of expansion. Thus the
Universe is not expanding into anything that we can see, and this is
not a profitable thing to think about. Just as Dali's Corpus
Hypercubicus is just a 2-D picture of a 3-D object that represents the
surface of a 4-D cube, remember that the balloon analogy is just a 2-D picture
of a 3-D situation that is supposed to help you think about a curved 3-D space,
but it does not mean that there is really a 4-D space that the Universe is
expanding into.

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What came before the Big Bang?

The standard Big Bang model is singular at the time of the Big Bang,
t = 0. This means that one cannot even define time, since spacetime is
singular. In some models like the chaotic or perpetual inflation favored by
Linde, the Big Bang is just one of many inflating bubbles in a spacetime foam.
But there is no possibility of getting information from outside our own one
bubble. Thus I conclude that: "Whereof
one cannot speak, thereof one must be silent."

From
Bruce Margon and Craig Hogan at the Univ. of Washington

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Why is the sky dark at night?
If the Universe were infinitely old, and
infinite in extent, and stars could shine forever, then every direction you
looked would eventually end on the surface of a star, and the whole sky would be
as bright as the surface of the Sun. This is known as Olbers' Paradox after
Heinrich Wilhelm Olbers [1757-1840] who wrote about it in 1823-1826 but it was
also discussed earlier. Absorption by interstellar dust does not circumvent this
paradox, since dust reradiates whatever radiation it absorbs within a few
minutes, which is much less than the age of the Universe. However, the Universe
is not infinitely old, and the expansion of the Universe reduces the accumulated
energy radiated by distant stars. Either one of these effects acting alone would
solve Olbers' Paradox, but they both act at once.

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Will the Universe expand forever or recollapse?
This depends on the
ratio of the density of the Universe to the critical density. If the density is
higher than the critical density the Universe will recollapse in a Big
Crunch. But current data suggest that the density is less than or equal to
the critical density so the Universe will expand forever. See Part 3 of the tutorial
for more information.

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Does entropy prevent a Big Crunch?

The issue of entropy in the Universe is subtle and not entirely settled.
Theorists are still trying to work out what happens to the entropy of matter
that falls into a black hole, a problem which involves both quantum mechanics
and strong gravity. When a successful theory of quantum gravity is worked out,
it should explain why the Universe came out of the Big Bang singularity with a
very large entropy, and what happens to the entropy of the Universe if it
recollapses.

Entropy is related to the number of ways a system can be in a given state or
condition. Thus a shuffled deck of cards has a higher entropy than a new deck
with all the suits in order. Adding energy to a system usually opens up more
states, and increases the entropy. The temperature of a system is defined such
that kT is the amount of energy needed to increase the number of available
states by a factor of e = 2.71828... where k is Boltzmann's constant.
Transferring heat from a hot piece of a system to a cold piece increases the
number of ways to arrange the cold part by a larger factor than the decrease in
the number of ways to arrange the hot piece. Thus the normal flow of heat from
hot to cold causes an increase in the number of ways the whole system can be
arranged which is then an increase in the total entropy of the whole system.

Entropy need not always increase in open systems. Energy could be used to
make entropy decrease for a particular system. Your refrigerator does this by
removing heat from the interior, if you consider the interior of your
refrigerator to be a separate system. Of course, if you consider both the
internal and external portions of the refrigerator then there is a net increase
of entropy due to the inefficiency of the refrigerator.

Since entropy is a statistical concept, short term fluctuations in small
systems can allow entropy to decrease.

Entropy remains constant in a system with a uniform temperature that has no
heat added or subtracted from it. This is thought to be more or less the case
for the Universe or for any representative piece of the Universe that expands or
contracts in the same way the Universe does. The vast majority of the entropy of
the Universe is in the cosmic microwave background radiation because the vast
majority of particles in the Universe are the photons of the CMB. As the
Universe expands, the temperature of the radiation drops to maintain constant
entropy. If the Universe were to collapse at some point, the radiation would
heat back up to maintain constant entropy. When the Universe expanded the
radiation started out in thermal equilibrium with the matter and then
de-coupled. In the collapse the radiation and the matter would once again come
into thermal equilibrium. Whatever happened with the dynamics of the matter in
the interim would be reflected in the final thermal equilibrium with the
radiation. The final entropy of the Universe as it approaches the Big Crunch
singularity would be larger than the initial entropy of the Universe because of
the heat added by nuclear fusion in stars, so a recollapse does not involve a
decrease in entropy.

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What about the oscillating Universe?
If the Universe recollapses, then
there is another singularity at the time of the Big Crunch. A singularity means
that the laws of physics break down, so we have no way to predict whether the
Big Crunch will connect to another cycle of expansion. Even if the density were
high enough to cause a recollapse, there would be no guarantee that the Universe
would oscillate. But the current evidence is strongly against any recollapse,
which would rule out the oscillating Universe. See PBS or
Ask an Astronomer
about this.

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What is the dark matter?

When astronomers add up the masses and luminosities of the stars near the
Sun, they find that there are about 3 solar masses for every 1 solar luminosity.
When they measure the total mass of clusters of galaxies and compare that to the
total luminosity of the clusters, they find about 300 solar masses for every
solar luminosity. Evidently most of the mass in the Universe is dark. If the
Universe has the critical density then there are about 1000 solar masses for
every solar luminosity, so an even greater fraction of the Universe is dark
matter. But the theory of Big Bang nucleosynthesis
says that the density of ordinary matter (anything made from atoms) can
be at most 10% of the critical density, so the majority of the Universe does not
emit light, does not scatter light, does not absorb light, and is not even made
out of atoms. It can only be "seen" by its gravitational effects. This "non-baryonic"
dark matter can be neutrinos, if they
have small masses instead of being massless, or it can be WIMPs (Weakly
Interacting Massive Particles), or it could be primordial black holes. My
nominee for the "least likely to be caught" award goes to hypothetical stable
Planck mass remnants of primordial black holes that have evaporated due to
Hawking radiation. The Hawking radiation from the not-yet evaporated primordial
black holes may be detectable by future gamma ray
telescopes, but the 20 microgram remnants would be very hard to detect.

Also see the Relativity
FAQ answer to this question, the
Cryogenic Dark Matter Search (CDMS) home
page, and Martin White on
dark matter.

See CDM

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What about MOND?
MOND, for Modification of Newtonian Dynamics, is an
alternative theory that tries to explain away the need for dark matter. While
some people are still trying to get it to work in cosmological models, it is not
widely
accepted.

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What is the value of the Hubble constant?

This is the question that professional astronomers ask the most frequently.
For many decades the Sandage [Ho = 50] and de Vaucouleurs
[Ho = 100] camps battled in a long lasting distance scale
controversy. Many outsiders thought that the geometric mean of these values,
Ho = 71 km/sec/Mpc, was a good compromise. The Sunyaev-Zeldovich
effect has been used to determine Ho = 77 +/- 10
km/sec/Mpc. The Hubble Space Telescope Key Project team came up with
the answer Ho =
72 +/- 8 km/sec/Mpc, the Cepheids in the nuclear maser
ring galaxy NGC 4258 gave 74 +/- 6 km/sec/Mpc, and the double-lined eclipsing binary in
M33 gave 61 +/- 4 km/sec/Mpc. These average to 71 +/- 5 km/sec/Mpc. Finally
WMAP came up with:

Ho = 71 +/- 3.5 km/sec/Mpc
based on a flat
Lambda-CDM model fit to the CMB angular power spectrum
but I would rather see

because it would be consistent with
a simple Universe with dark matter but no dark energy. But the supernova
evidence for an accelerating Universe makes this rather unlikely, even if the
Hubble constant data were wrong.

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What can a layperson do in cosmology?

Stay in school! There is a lot to learn about the Universe.

Keep taking math and science courses!
The book of nature lies
continuously open before our eyes (I speak of the Universe) but it can't be
understood without first learning to understand the language and characters in
which it is written. It is written in mathematical language, and its
characters are geometrical figures. - Galileo Galilei
That was true
400 years ago and it is much more true today!

If you are out of school, check out the bibliography.

Tell your Congressman and
Senators to support astrophysics research at NASA, NSF, and DOE.

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When is the next WMAP data release?

The data release at 1300 UTC on 5 Mar 2008 included the first 5 years of
data. The hourglass has been
turned over. There will be further releases but the schedule is not yet known.

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Ned Wright's Home
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Tutorial : Part 1 | Part 2 | Part 3 | Part 4 | Age | Distances | Bibliography | Relativity

© 1996-2008 Edward
L. Wright. Last modified 07 April 2008