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Physics LibreTexts

17.8 Cosmic Microwave Background Radiation

 


 

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The Cosmic Microwave Background, mapped from WMAP. Click here for original source URL.

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Robert Wilson, left, and Arno Penzias stand in front of the Bell Labs horn radio antenna in Crawford Hill, N.J., where they discovered cosmic background radiation confirming the Big Bang. Click here for original source URL.

The cosmic microwave background (CMB) radiation is a crucial piece of evidence that supports the big bang model for the origin of the universe. First observed accidentally by the radio engineers Arno Penzias and Robert Wilson in 1965, the CMB is a diffuse, uniform background of microwave radiation that comes from all directions in the sky. Astronomers have sought progressively more detailed observations of the CMB, using satellites and balloons to get above the bulk of the Earth's atmosphere. Three space missions in particular have been instrumental in measuring this radiation in finer and finer detail and with better and better precision. NASA's Cosmic Background Explorer (COBE) took data from 1989 to 1996 and it was the first to detect tiny variations in the intensity of the radiation. Next came the Wilkinson Microwave Anisotropy Probe (WMAP), launched by NASA in 2001. WMAP took data for a decade and its very accurate measurements put constraints on a set of parameters of the bib bang model. The current state-of-the-art experiment is ESA's Planck mission, launched in 2009. The level of accuracy of Planck data is tens of thousands of times greater than the discovery measurements of Penzia and Wilson.

The microwave background radiation has two striking features. First, the spectrum is thermal, with the data lying with great accuracy along a black body curve that defines a temperature of 2.726 K. Note that we can quote the temperature to four significant figures and an accuracy of a thousandths of a degree — it is the most accurately determined number in cosmology. The cosmic background radiation has the most perfect thermal spectrum observed anywhere in nature. Since astronomers believe that the radiation dates from about 380,000 years after the big bang, this means that the universe was in thermal equilibrium at that time. Second, the radiation is almost totally isotropic, meaning that it has the same intensity in all directions. If we represent the intensity of the microwave radiation by the surface of a pond 100 meters across, then the biggest ripples are only a few millimeters high. These small fluctuations are generated by minute differences in density in the early universe; differences that lead to the eventual formation of structures and voids.

When we observe the CMB, we have to correct it for our own motion, including; the Sun's motion around the center of the galaxy, the galaxy's motion in the Local Group, and the Local Group's motion toward the Virgo cluster. The background is slightly warmer, or blue shifted, in the direction we are moving (toward the constellation Leo) and slightly cooler, or red shifted, in the opposite direction (toward the constellation Aquarius). Our motion is 371 km/s toward Leo, which translates into 0.0035 K hotter radiation in this direction and 0.0035 K cooler radiation in the direction of Aquarius.

The CMB shines on us from all directions. This does not mean we are at the center of the universe, rather it means the CMB was released from every point in space nearly 13.7 billion years ago. If we traveled to the other edge of visible universe, we'd still see the radiation, but the photons we'd be seeing would have originated from a different place in space. Here is a way to think of this. If you swim under water at night, your headlight will illuminate a set sphere of material around your head, and as you swim the volume of water you can see will always be the same size sphere, but the sphere you see will change. With the CMB, as we move we see photons that originated from different places (although a massive, and impossible, movement would be required to see an actual change), but we always seem to be at the center of our own sphere of CMB radiation. Another way to say this is that the big bang was the creation of all space, so its relic radiation fills space — the big bang is everywhere.

The CMB permeates all of space, and the energy it generates is something that has to be considered when scientists consider how the universe will end, and how long it takes black holes to evaporate. In every breath you take, you inhale 100,000 or so of these ancient photons. The photons are so feeble that there is no health hazard posed by this radiation; the power is only 10-5 Watts, or a ten millionth of the luminous intensity of a light bulb. To see this radiation for yourself, tune an old TV (one with a tube rather than a flat screen) between stations. About 1% of the noise specks on the screen are due to interactions with the cosmic background radiation. Evidence of the Big Bang is all around us.
 

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Graph of cosmic microwave background spectrum measured by the FIRAS instrument on the COBE satellite, the most-precisely measured black body spectrum in nature, the error bars are too small to be seen even in enlarged image, and it is impossible to distinguish the data from the theoretical curve. Click here for original source URL