Quick
Navigation Quick Nav
Menu Home
Search Status News Technical Site Map Links Glossary Image Gallery About MAP

How Fast is the Universe Expanding?

Historical Overview

In the 1920s, Edwin Hubble, using the newly constructed 100"
telescope at Mount Wilson Observatory, detected variable stars in
several nebulae. Nebulae are diffuse objects whose nature was a
topic of heated debate in the astronomical community: were they
interstellar clouds in our own Milky Way galaxy, or whole galaxies
outside our galaxy? This was a difficult question to answer because
it is notoriously difficult to measure the distance to most
astronomical bodies since there is no point of reference for
comparison. Hubble's discovery was revolutionary because these
variable stars had a characteristic pattern resembling a class of
stars called Cepheid variables. Earlier, Henrietta Levitt, part of a
group of female astronomers working at Harvard College Observatory,
had shown there was a tight correlation between the period of a
Cepheid variable star and its luminosity (intrinsic brightness). By
knowing the luminosity of a source it is possible to measure the
distance to that source by measuring how bright it appears to us:
the dimmer it appears the farther away it is. Thus, by measuring the
period of these stars (and hence their luminosity) and their
apparent brightness, Hubble was able to show that these nebula were
not clouds within our own Galaxy, but were external galaxies far
beyond the edge of our own Galaxy.

Hubble's second revolutionary discovery was based on comparing
his measurements of the Cepheid-based galaxy distance determinations
with measurements of the relative velocities of these galaxies. He
showed that more distant galaxies were moving away from us more
rapidly:

v = Hod

where v is the speed at
which a galaxy moves away from us, and d is its distance. The
constant of proportionality Ho is now called the Hubble
constant. The common unit of velocity used to measure the speed of a
galaxy is km/sec, while the most common unit of for measuring the
distance to nearby galaxies is called the Megaparsec (Mpc) which is
equal to 3.26 million light years or 30,800,000,000,000,000,000 km!
Thus the units of the Hubble constant are (km/sec)/Mpc.

The universe was not static, but rather was expanding! This
discovery marked the beginning of the modern age of cosmology.
Today, Cepheid variables remain one of the best methods for
measuring distances to galaxies and are vital to determining the
expansion rate (the Hubble constant) and age of the universe.

What are Cepheid Variables?

The structure of all stars, including the Sun and Cepheid
variable stars, is determined by the opacity of matter in the star.
If the matter is very opaque, then it takes a long time for photons
to diffuse out from the hot core of the star, and strong temperature
and pressure gradients can develop in the star. If the matter is
nearly transparent, then photons move easily through the star and
erase any temperature gradient. Cepheid stars oscillate between two
states: when the star is in its compact state, the helium in a layer
of its atmosphere is singly ionized. Photons scatter off of the
bound electron in the singly ionized helium atoms, thus, the layer
is very opaque and large temperature and pressure gradients build up
across the layer. These large pressures cause the layer (and the
whole star) to expand. When the star is in its expanded state, the
helium in the layer is doubly ionized, so that the layer is more
transparent to radiation and there is much weaker pressure gradient
across the layer. Without the pressure gradient to support the star
against gravity, the layer (and the whole star) contracts and the
star returns to its compressed state.

Cepheid variable stars have masses between five and twenty solar
masses. The more massive stars are more luminous and have more
extended envelopes. Because their envelopes are more extended and
the density in their envelopes is lower, their variability period,
which is proportional to the inverse square root of the density in
the layer, is longer.

Text Link to the HST
press release describing this image.

Difficulties in Using Cepheids

There have been a number of difficulties associated with using
Cepheids as distance indicators. Until recently, astronomers used
photographic plates to measure the fluxes from stars. The plates
were highly non-linear and often produced faulty flux measurements.
Since massive stars are short lived, they are always located near
their dusty birthplaces. Dust absorbs light, particularly at blue
wavelengths where most photographic images were taken, and if not
properly corrected for, this dust absorption can lead to erroneous
luminosity determinations. Finally, it has been very difficult to
detect Cepheids in distant galaxies from the ground: Earth's
fluctuating atmosphere makes it impossible to separate these stars
from the diffuse light of their host galaxies.

Another historic difficulty with using Cepheids as distance
indicators has been the problem of determining the distance to a
sample of nearby Cepheids. In recent years, astronomers have
developed several very reliable and independent methods of
determining the distances to the Large Magellanic Cloud (LMC) and
Small Magellanic Cloud (SMC), two of the nearby satellite galaxies
of our own Milky Way Galaxy. Since the LMC and SMC contain large
number of Cepheids, they can be used to calibrate the distance
scale.

Recent Progress

Recent technological advances have enabled astronomers to
overcome a number of the other past difficulties. New detectors
called CCDs (charge coupled devices) made possible accurate flux
measurements. These detectors are also sensitive in the infrared
wavelengths. Dust is much more transparent at these wavelengths. By
measuring fluxes at multiple wavelengths, astronomers were able to
correct for the effects of dust and make much more accurate distance
determinations.

These advances enabled more accurate study of the nearby galaxies
that comprise the "Local Group" of galaxies. Astronomers observed
Cepheids in both the metal rich inner region of the Andromeda galaxy
and its metal poor outer region. (To an astronomer, a "metal" is any
element heavier than helium - the second lightest element in the
periodic table. Such elements are produced in stars and are
ultimately released into the interstellar medium as the stars
evolve.) This work showed that the properties of Cepheids did not
depend sensitively on chemical abundances. Despite these advances,
astronomers, limited by the Earth's atmosphere, could only measure
the distances to the nearest galaxies. In addition to the motion due
to the expansion of the universe, galaxies have "relative motions"
due to the gravitational pull of their neighbors. Because of these
"peculiar motions", astronomers need to measure the distances to
distant galaxies so that they can determine the Hubble constant.

Trying to push deeper into the universe, astronomers have
developed a number of new techniques for determining relative
distances to galaxies: these independent relative distance scales
now agree to better than 10%. For example, there is a very tight
relation, called the Tully-Fisher relation, between the rotational
velocity of a spiral galaxy and its luminosity. Astronomers also
found that Type Ia supernova, which are thought to be due to the
explosive burning of a white dwarf star, all had nearly the same
peak luminosity. However, without accurate measurements of distance
to large numbers of prototype galaxies, astronomers could not
calibrate these relative distance measurements. Thus, they were
unable to make accurate determinations of the Hubble constant.

Over the past few decades, leading astronomers, using different
data, reported values for the Hubble constant that varied between 50
(km/sec)/Mpc and 100 (km/sec)/Mpc. Resolving this discrepancy, which
corresponds to a factor 2 uncertainty, was one of the most important
outstanding problems in observational cosmology.

Hubble Key Project

One of the "key projects" of the Hubble Space Telescope is to
complete Edwin Hubble's program of measuring distances to nearby
galaxies. While the Hubble Space Telescope (HST) is comparable in
diameter to Hubble's telescope on Mount Wilson, it has the advantage
of being above the Earth's atmosphere, rather then being located on
the outskirts of Los Angeles. NASA's repair of the Hubble Space
Telescope restored its vision and enabled the key project program.
The photos below show before and after images of M100, one of the
nearby galaxies observed by the key project program. Note that with
the refurbished HST, it is much easier to detect individual bright
stars in M100, a necessary step in studying Cepheid variables. The
project also checks to see if the properties of Cepheid variables
are sensitive to stellar composition.

HST image of M100 before and after repair

Text Link to the HST press release describing this
image.

Overall, the key project aims to get distances to 20 nearby
galaxies. With this large sample, the project can calibrate and
cross check a number of the secondary distance indicators. Because
M100 is close enough to us that its peculiar motion is a significant
fraction of its Hubble expansion velocity, the key project team used
relative distance indicators to extrapolate from the Virgo cluster,
a nearby cluster of galaxies containing M100, to the more distant
Coma cluster and to obtain a measurement of the Hubble constant of 70
(km/sec)/Mpc, with an uncertainty of 10%.

The key project determination of the Hubble constant is
consistent with a number of independent efforts to estimate the
Hubble constant: a recent statistical synthesis by G.F.R. Ellis and
his collaborators of the published literature yields a value between
66 and 82 (km/sec)/Mpc. However, there is still not complete
consensus on the value of the Hubble constant: a recent analysis by
Allan Sandage using Type Ia supernovae yields a value for the Hubble
constant that is formally inconsistent with many of measurements: 47
(km/sec)/Mpc.

WMAP and the Hubble Constant

By characterizing the detailed structure of the cosmic microwave
background fluctuations, WMAP will be able
to accurately determine the basic cosmological parameters,
including the Hubble constant, to better than 5%. This measurement
is completely independent of traditional measurements using Cepheid
variables and other techniques.
The initial results show the Hubble Constant to be 71 (km/sec)/Mpc,
+0.04/-0.03.

This page was adapted from the article "The age of the
universe", D.N. Spergel, M. Bolte (UC, Santa Cruz) and W.
Freedman (Carnegie Observatories). Proc. Natl. Acad. Sci. USA, Vol.
94, pp. 6579-6584, June 1997.

Further Reading:

More
on the Hubble Constant from Space Telescope Science Institute
including movies.

Freedman, Wendy L., "The Expansion Rate and Science of the
Universe", Scientific American, Nov. 1992.

Osterbrock, D.E., Gwinn, J.A. & Brashear, R.S., "Hubble
and the Expanding Universe", Scientific American, July 1993.

Overview
Matter
Expansion
Age
Shape
Acceleration
Fate

Last updated: Thursday, 01-08-2004

Back
to Top

Responsible
NASA Official: Charles Bennett   Webmaster: Britt
Griswold
Comments &
Suggestions - Privacy/Security
Warning

Overview

Matter

Expansion

Age

Shape

Acceleration

Fate