The Big Bang & The Early Universe

Line of Evidence #1 We have discussed the accelerating expansion of the Universe and the ultimate fate of everything. If we “play the movie backwards”, we find that the Universe was denser and hotter in the past. If we look back far enough, the Universe must have started out incredibly dense and hot. This consequence of Hubble’s cosmic expansion was realized first by Georges Lemaitre, a Belgian astronomer and Catholic priest. Today, we call this theory of the beginning of the Universe “the Big Bang theory”.

Back to the Big Bang Cosmic expansion means that the early Universe was hotter and denser than it is today. The future Universe will be less dense and cooler than today.

The Earliest Moments The conditions at the start of cosmic history are so extreme, they are beyond the reach of modern particle physics: infinite temperature and density However, the Universe expanded and cooled quite rapidly and by 10-43 seconds after the Big Bang the Universe had a temperature of 1032 Kelvin. At this time, our current understanding of physics allows us to ponder what the Universe was like.

Formation of Matter & Antimatter By the time the Universe was 10-38 seconds old and around 1039 Kelvin, our current theories of electromagnetism, the strong and weak force can describe what the Universe was like. At that time, particles formed and annihilated over and over. Their masses sprang from energy and returned once again (recall E = mc2).

Formation of Matter & Antimatter A long-standing mystery in cosmology is the absence of antimatter from our Universe today. One theory published in November 2010 suggests that dark matter is actually the missing antimatter. Here is another situation that may be solved by data from particle experiments like LHC.

Once the Universe was 1-millionth of a second old, all matter (normal + dark) had formed.

Almost a Universe We’d Recognize By 10-3 seconds after the Big Bang, the Universe had cooled to a temperature of 1012 Kelvin! By now, all four fundamental forces of nature (gravity, electromagnetism, strong, weak nuclear) all behaved as they do today. At this time, protons and neutrons had formed and began fusing to form light nuclei: • deuterium (heavy hydrogen, 1 proton + 1 neutron) • helium-3 (2 protons + 1 neutron) • helium-4 (2 protons + 2 neutrons) • lithium-7 (3 protons + 4 neutrons)

Formation of the Light Elements Over the next few hundred seconds, the Universe continued to cool and expand. But eventually the density dropped low enough that reactions were too slow to add many more nuclei. The period of nucleosynthesis that established the original chemical composition of the Universe lasted about 3 minutes. Calculations of this period of nucleosynthesis predicts that the Universe should have started with about 75% hydrogen and 25% helium.

Line of Evidence #2 Observations of the earliest stars and gas clouds indicate a chemical composition very close to that 75% : 25% ratio of hydrogen to helium. All other elements were produced in the nuclear reactions in the cores of stars and supernovas.

The chemical history of the Universe is evidence of a hot, dense Big Bang at the beginning.

A Cosmic Fog For the next 380,000 years, the Universe continued to cool and expand. During this time, the temperature was too high for any atoms to form. All matter consisted of free nuclei & electrons, just like in the interior of a star. Just like the inside of a star, the Universe at this time was opaque to electromagnetic waves. Eventually, the temperature dropped to 3000 K and hydrogen and helium atoms formed across the Universe for the first time.

Hot Enough to Glow In 1948, Ralph Alpher and George Gamov wrote an article predicting a cosmic radio afterglow from the Big Bang. They realized that the early Universe must have been extremely hot. It glowed as a blackbody. Initially, this glow had a short-wavelength peak because of the high temperature. But cosmic expansion would have stretched the peak wavelength into the radio part of the spectrum.

Cosmic Microwave Background Their paper was essentially forgotten after it was published since radio astronomy was still in its infancy in the 1940’s. The paper was rediscovered in the early 1960’s by American and Soviet astronomers who were building more sophisticated radio telescopes. A team led by Robert Dicke at Princeton University began building a sensitive radio telescope to search for this cosmic afterglow.

Cosmic Microwave Background At the same time, Arno Penzias and Robert Wilson of Bell Labs in New Jersey found that the sky was filled with a uniform radio glow with a temperature of 3.5 Kelvin. They thought they were receiving interference from some source on Earth. In 1965, Penzias and Wilson met with the Princeton team of astronomers to discuss the mysterious source of radio emission. Dicke and the other astronomers immediately recognized the emission as the radio afterglow of the Big Bang predicted by Alpher and Gamow.

Cosmic Microwave Background In 1978, Penzias (right) and Wilson (left) won the Nobel Prize in Physics for their discovery.

The lesson here is sometimes you get lucky. You can win a Nobel just for being the first to have a cool new telescope!

Cosmic Microwave Background Over the years, improved radio telescopes have measured this cosmic microwave background. In 1989, the COBE satellite measured the spectrum of the microwave background to unprecedented accuracy. At a January 1990 meeting, George Smoot of the COBE team presented the first 9 minutes of data from the new satellite. In that short observing time, COBE had confirmed the prediction of a perfect blackbody spectrum, just as Gamow and Alpher wrote 40 years earlier.

Line of Evidence #3 An exact temperature of 2.725 Kelvin was found. The spectrum confirmed the Big Bang precisely.

Cosmic Microwave Background Here is the COBE all-sky map. The temperature across the entire sky is the same: 2.725 Kelvin. Well, almost exactly. If we display the map to enhance small temperature differences…

Cosmic Microwave Background We see the CMB is hotter and brighter on one side of the sky (red) than the other (blue). (BACKWARDS!) This is due to the combined motion of the Sun in the Milky Way, the Milky Way in the Local Group, and the Local Supercluster through space.

Moving toward this spot in the sky.

Cosmic Microwave Background The microwave background was emitted so early in the history of the Universe that matter had little time to start moving due to gravity as clouds collapsed to form superclusters.

If Earth were not moving through space, we would see no Doppler shift at all.

Cosmic Microwave Background But our motion through space (about 600 km/s) causes the cosmic microwave background to appear brighter and hotter on one side of the sky than on the other.

Because we are moving, we see a brighter glow on one side of the sky compared to the other.

Cosmic Microwave Background After removing the Doppler shift due to our motion and the microwave emission from dust and gas in the Milky Way, the map still has tiny variations in temperature: +/- 0.00003 Kelvin (30 microKelvin).

Cosmic Microwave Background NOTE: the color coding in these maps is backward compared to a blackbody: (red v. blue, hot v. cold) COBE scientists chose to use “faucet” colors so non-astronomers would understand.

Cosmic Microwave Background These tiny variations in temperature are due to slightly warmer or cooler gas in different regions of the Universe as larg scale structure formed. George Smoot and John Mather won the 2006 Nobel Prize in Physics for their work on COBE.

Cosmic Microwave Background A new satellite called WMAP began observing the cosmic microwave background in late 2001. Since then, WMAP has mapped the brightness of the cosmic microwave background across the entire sky several times. WMAP began by confirming COBE’s earlier finding that the cosmic microwave background was not perfectly uniform and had a temperature of 2.7 K. But WMAP’s antennas provided much higher resolution on the sky, so it was able to observe much smaller variations in the brightness of the microwave background.

Cosmic Microwave Background Here are maps comparing the COBE & WMAP observations. Color coding is the same in both, but notice COBE could not see the same detail that WMAP can. Again, colors are backwards…

7 degree resolution

0.3 degree resolution

Cosmic Microwave Background The variations in the microwave background are the glow from the higher-density regions that collapsed to form superclusters of galaxies.

Cosmic Microwave Background Why does higher density mean brighter emission? Recall that a cloud that is collapsing also heats up due to gravitational work. Those regions that are higher density are also hotter. We’ve also discussed how the brightness of a blackbody increases with temperature.

Brighter, hotter emission means high density gas.

Fainter, cooler emission means low density gas.

MORE MATTER HERE

LESS MATTER HERE

Measuring the Cosmic Recipe In January 2010, the analysis of 7 years of WMAP data was released. Combined with data from other experiments, they provide an accurate measure of the Universe: Age of Universe: 13.75 +/- 0.11 billion years Age at CMB formation: 380,000 +/- 5800 years Hubble Constant: 70.2 +/- 1.4 km/s per Mpc

Cosmic History Cosmic Microwave Background

The Accelerating Universe

Big Bang

First stars Formation of galaxies Today

Foundations of Modern Cosmology More details and concepts have been added, but the standard theory is still founded on three basic ideas: 1. The expansion of the Universe & Hubble’s Law of the motion of distant galaxies 2. The prediction of nuclear fusion throughout the Universe during the “first three minutes” & the observed abundances of H/He in early objects 3. The epoch when the Universe cooled and atoms formed & the observed spectrum and properties of the cosmic microwave background

When did the first atoms form after the Big Bang? A. Within seconds of the beginning B. About five minutes after the Big Bang C. About 500,000 years after the Big Bang D. About 1 billion years after the Big Bang

According to the Big Bang theory, when did the cosmic microwave background form? A. Within seconds of the beginning B. About five minutes after the Big Bang C. About 500,000 years after the Big Bang D. About 1 billion years after the Big Bang

If astronomers observed a much slower expansion to the Universe than they really do, they would also predict that the period of nucleosynthesis would last: A. much longer B. about the same C. much shorter

Which of the following observations supports the idea that the early Universe was nearly the same density everywhere? A. the cosmic microwave background has a blackbody temperature of 2.7 Kelvin B. the cosmic microwave background is lumpy on the scale of 1 part in 100,000 C. distant galaxies all have spectra which indicate about 75% H and 25% He D. distant galaxies all have redshifted spectra

According to the Big Bang theory, what was the entire Universe like about 2 minutes after the beginning? A. super hot, nearly 1020 Kelvin, too hot for protons and neutrons to exist B. hot, about 108 Kelvin, like the core of a star C. warm, just under 106 Kelvin, too hot for atoms D. cool, about 103 Kelvin, hydrogen and helium existed throughout the Universe

According to the Big Bang theory, what was the entire Universe like about 2 million years after the beginning? A. super hot, nearly 1020 Kelvin, too hot for protons and neutrons to exist B. hot, about 108 Kelvin, like the core of a star C. warm, just under 106 Kelvin, too hot for atoms D. cool, about 103 Kelvin, hydrogen and helium existed throughout the Universe