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 Cosmic Observations observations. Background of galaxies of Theories predict Radiation. different ages. the state and development of (directly Cosmic Background Radiation the Universe. observed) redshifted to microwave wavelengths. Big Bang Recombination Present Day t = 0 t ≈ 400,000 yr 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 W ≈ 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 .