Astronomy C10/L&S C70U
Nicholas McConnell, GSI
The Cosmic Microwave Background Radiation: Our Best Window to the Early Universe
Given that we see distant objects as they were in the past, can we hope to find
increasingly distant objects until we can directly observe the Big Bang?
Although more distant objects do allow us to observe the Universe further back in
time, we cannot see all the way back to the Big Bang. This is because the Universe was
opaque for the first few hundred thousand years of its existence. Instead of traveling long
distances like the light from other galaxies does to reach us, light in the early Universe
was constantly deflected by charged particles.
The earliest feature of our Universe that we can observe directly is the Cosmic
Background Radiation, a "glow" of highly redshifted light in all directions of the sky.
This radiation comes from the period when the Universe transitioned from being opaque
to being transparent.
No direct
observations.
Theories predict
the state and
development of
the Universe.
Big Bang
t=0
Cosmic
Background
Radiation.
(directly
observed)
Observations
of galaxies of
different ages.
Cosmic Background Radiation
redshifted to microwave wavelengths.
Recombination
t ≈ 400,000 yr
Present Day
t ≈ 13.7 billion yr
What caused the Cosmic Microwave Background Radiation?
The Cosmic Microwave Background Radiation (CMBR, or CMB, or CBR--all
abbreviations refer to the same thing) was caused by "Recombination" when the Universe
was about 400,000 years old. At this time, the Universe had expanded and cooled to the
point where electrons and protons could bind together and form atoms. Each "capture" of
an electron by a proton releases a photon. Furthermore, neutral atoms scatter light much
less efficiently than charged particles, so the Universe quickly became transparent,
allowing the newly created photons to travel long distances.
Now we can observe this radiation from all areas of the Universe, just as we can
observe galaxies that are billions of years old. However, the expansion of the Universe
between the time of Recombination and now has redshifted the CMBR, so that we
receive low-energy microwave light, instead of visible or ultraviolet light.
What can the Cosmic Microwave Background Radiation tell us about our Universe?
The image above is a map of the CMBR from WMAP, a successful satellite
mission that ran from 2001 to 2003. The red and blue granulation indicates very small
variations in the temperature associated with the CMBR. Careful measurements of the
size of these variations can tell us a number of things, including the age of the Universe
(13.7 ± 0.5 billion years) and the current value of Hubble's constant (71 ± 4 km/s/Mpc).
Assuming we have good predictions for the physical diameter of the variations in
the CMBR, the angular size that we measure will also tell us about the geometry of the
Universe. Because the measured angles neither underestimate nor overestimate the
diameter of the variations, we conclude that we live in a nearly (if not perfectly) flat
universe, with ≈1 ± 0.02.
More info: http://map.gsfc.nasa.gov/
Just the picture: http://apod.oa.uj.edu.pl/apod/image/0302/sky_wmap_big.jpg
How is the Cosmic Microwave Background Radiation related to the appearance of
the Universe today?
The temperature variations in the CMBR correspond to density variations of
matter in the Universe at the time of Recombination. As the Universe continued to age,
gravity helped denser regions become denser, and expansion stretched the less dense
regions in to open voids. Eventually, stars, star clusters, galaxies, and galaxy clusters
formed where matter was densest.
The left image below is from the Millennium Simulation. In 2005, physicists
began with the conditions observed in the CMBR and ran a computer simulation to see
how the laws of gravity would cause matter to arrange itself over time. At the present
age of the Universe, the simulation showed a spiderweb-like structure, with enormous
filaments of matter (note the scale of 125 Mpc in the picture) stretching across space.
The small yellow dots along the filaments and their intersections are individual galaxies
and galaxy clusters.
The right-hand image is the actual distribution of nearby galaxies, based on
observations from the Sloan Digital Sky Survey, an automated telescope survey. Each
tiny black dot in this image is a galaxy, at its position in the sky and distance (the Milky
Way is at the center of the Circle). Note that the galaxies arrange themselves into
filaments, very similar to the simulation's picture!!! The similarity between these two
images demonstrates how the initial conditions imprinted in the Cosmic Microwave
Background Radiation actually gave rise to the evolution of our Universe, resulting in the
distribution of galaxies we observe today.
Millennium Simulation Sites: http://www.virgo.dur.ac.uk/new/index.php?subject=millennium
http://www.mpa-garching.mpg.de/galform/millennium/
Big Image: http://www.mpa-garching.mpg.de/galform/millennium/seqD_063a_half.jpg
SDSS Site: http://www.sdss.org/
SDSS Image: http://spiff.rit.edu/richmond/sdss/sn_survey/galaxy_map.gif