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조지 가모프 (1904--1968)은 1948년에 우주가 지금보다 더 뜨겁고 밀도가 높던 시절부터 점차 식어 가면서 남아 있는 희미한 빛이 있어야 한다고 예측했다. 우주가 팽창하는 것으로 관측이 된다면 그것은 은하들이 예전에는 한 곳이 몰려 있었고 우주의 에너지도 지금보다 작은 부피에 집중되어 있었다는 것을 의미한다. 우주의 초창기에는 감마선이 우주를 돌아 다니고 있었을 것이며 좀더 팽창한 후에는 x선으로 우주가 빛났을 것이다. 우주 탄생후 150억년이 지난 지금에는 그 빛이 전파 영역이 되어야 한다. 팽창 속도는 중력의 영향으로 시간이 지나면서 점점 느려지게 되었다. 즉 예전에는 우주가 현재보다 팽창 속도가 빨랐다는 이야기인데 우주 탄생때에는 팽창의 속도가 매우 빨랐다.
프레드 호일은 우주 초기의 빠른 팽창 속도와 높은 온도를 보고 우주 탄생에 관한 이론을 빅뱅이라고 불렀다. 호일이 이 용어를 처음 만들 때 정적우주이론이라고 불리는 완벽한 우주 원리를 사용하는 다른 이론을 옹호하고 있었다. 그래서 그 때는 호일의 '빅뱅'이라는 용어는 사람들이 쓰기를 꺼려했다. 그러나 빅뱅을 주장하는 사람들은 그 용어를 좋아했고 지금까지 그것이 쓰이고 있다.
Observation
Arno Penzias and Robert Wilson observed in 1965 a radio background source that was spread all over the universe---the cosmic microwave background radiation. The radiation has the same intensity and spectral character as a thermal continuous source at 3 K (more precisely, 2.728 ?0.004 K) as measured by the COBE satellite in every direction observed. To a high degree of precision the sky is uniformly bright in radio. The uniformity of the background radiation is evidence for the cosmological principle. The error bars in the figure below are too small to be seen.
Interpretation
This background radiation is interpreted to be the relic of the early universe. If this is correct, then the early universe was very uniform. Since the further out in space you look means the further back in time you look, the microwave radiation is coming from the universe as it was a few hundred thousand years after the Big Bang when the universe was much hotter. The glow from the early hot universe has been redshifted by 1000 times! Hoyle's Steady State theory could not adequately explain the presence of the background radiation and so was abandoned by most astronomers.Let's take a closer look at what was happening in the universe when it produced the background radiation. The early universe (both the matter and the radiation) was much more compact. The radiation density was so great that it dominated the expansion rate and the conditions of the universe for the first 10,000 years. Remember Einstein's equation relating energy and mass? The energy E=mc2 so the radiation energy had a definite gravitational effect!
The early universe was hot and opaque (photons could not move very far before being absorbed). The freely-moving electrons, protons, and neutrons scattered the photons all about making the dense gas opaque. Dense hot gases will produce a continuous spectrum that depends only on the temperature (a thermal spectrum). The universe cooled off as it expanded. Eventually, the early universe cooled to where the electrons and protons could combine to form neutral hydrogen atoms and not be blown apart by energetic photons. The process of the electrons becoming bound to the protons to make atoms is called recombination. Okay, ``recombination'' is not really correct since this was the first time that the electrons combined with the protons, but it also describes processes that occur today.
At the time of recombination, the number of unit particles was cut at least in half (one electron + one proton become a single atom; the neutrons also were incorporated into the atoms). That meant the photons could travel further without hitting some kind of unit particle. Also, the expansion of the universe spread the matter out. In addition, the coolness of the universe (only 3000 K at the time of recombination) meant that longer wavelengths of light were present. Instead of the gamma rays and X-rays of earlier times, the predominant form of radiation was the longer wavelength visible light and infrared. Longer wavelengths of light are able to more easily to pass through gas. For all of these reasons the photons could then travel long distances without running into some particle. The universe became transparent when the universe was glowing at the temperature of the surface of a cool star.
Extrapolating the expansion rate and the temperature of the universe backward in time, one finds that at the temperature of 3000 K, the universe was a few hundred thousand years old. The photons from this time are now reaching our radio telescopes. They are by far the oldest radiation that can be detected.
The universe could not have been perfectly uniform, though. The universe must have been slightly lumpy to form galaxies and people later on from the internal gravity of the lumps. Gravity is symmetrical so it needed some initial density variations to provide some direction to where surrounding matter could be attracted. The COBE satellite found slight variations in the brightness of the background radiation of about 1 part in 100,000. The slight variations exist because some parts of the universe were slightly denser than other parts. The slightly denser regions had more gravity and attracted more material to them while the expansion occurred. Over time, the denser regions got even denser and eventually formed galaxies about 1 billion years after the Big Bang.
Below is a sequence of false-color microwave all-sky maps from the Differential Microwave Radiometer (DMR) instrument on the COBE satellite. The galactic equator runs horizontally through the center of each map. The range of temperatures for each map is given in the caption.
 The colors for the temperatures range from blue for 0 K to red for 4 K (yes, the color scheme is backward---blue should be hot and red should be cool). Notice that the background appears completely uniform at a temperature of 2.728 K. |
 The colors for the temperatures range from blue for 2.724 K to red for 2.732 K. The double-lobe pattern shows the doppler effect from the motion of the Sun with respect to the background radiation. The background appears about 1/1000 times hotter (redder in this false-color map) in the direction the Sun is moving toward and about 1/1000 times cooler (bluer here) in the direction the Sun is moving away from. |
The colors for the temperatures range from blue for 2.7279 K to red for 2.7281 K. The effect of the Sun's motion has been subtracted out leaving fluctuations that are thirty times smaller than the previous map. The faint microwave contribution of the Milky Way is clearly seen along the center. The Cygnus constellation is at left center, the Sagittarius constellation is at the center, and the Orion constellation is at right center. |
Below is a picture of the fluctuations in the background radiation when the Milky Way's contribution is subtracted out. The view is now looking up above the galactic plane and looking down below the galactic plane. The largest superclusters, such as the ``Great Wall'' seen in Geller and Huchra's slice map of our local part of the universe, would easily fit inside the smallest feature on this map. Selecting the map will take you to more DMR images from the COBE mission in another window.
