Santanu Das

Do they measure the light from Big Bang correctly?

  • November 28, 2015
Do they measure the light from Big Bang correctly?
When you look into the night sky, you look back in time. Moonlight, which is actually the light from Sun bounced off the lunar surface nearly two seconds ago we see it. Light from the bright star Vega left about 26 years ago. And the light from the Andromeda galaxy began its journey to our telescopes about 2 million years ago.
But how far back in time can you see? If we had a big enough telescope, could we see the first stars and galaxies? And could you see the light from the great flash of the Big Bang itself? The answer to all these questions is yes. With radio telescopes, astronomers have detected faint light that fills the universe, and which has all the telltale signs of the first light to emerge from the hot universe just 380,000 years after the Big Bang.
Right after the Big Bang, the universe was hot… millions of degrees Kelvin. As the universe expanded, it cooled. But for hundreds of thousands of years it was still too hot for protons and electrons to combine into hydrogen. So the early universe was a hot glowing fog of charged subatomic particles that scrambled photons in all directions.
Then some 380,000 years after the Big Bang the temperature of the universe dropped to about 3,000 Kelvin, cool enough for protons and electrons to settle down as hydrogen atoms. With the electrons locked up, photons were free to spray forth through the cosmos, and the young universe was filled with dull red and infrared light.  We can still detect the vestiges of this light today.
This “first light” was a major prediction of the Big Bang Theory. In the 1940’s, physicists Ralph Alpher and Robert Herman reasoned this light is now stretched to longer wavelengths by the expansion of space itself. A simple calculation showed the light waves are stretched by a factor of 1000, which means the infrared light were now microwaves corresponding to a glowing body of temperature about 3 K (three degrees above absolute zero).  
 In 1965 two scientists at Bell Labs in Crawford Hill, New Jersey, Arno Penzias and Robert Wilson, detected the cosmic microwave background (CMB) accidentally while working on a antenna for commercial satellite communications.
The temperature of the CMB is amazingly uniform across the sky, which means the early universe was amazingly smooth. But there are small variations of about 2 parts in 10,000 (i.e. fluctuations in micro K order) in the CMB, variations which are also predicted by detailed calculations of the Big Bang. These slight variations have huge implications for the evolution of the universe from a hot soup of smooth gas to the lumpier stars and galaxies.
These temperature fluctuations in the CMB originated due to a fact called the gravitational redshift. When a photon comes from a region with higher gravitational potential, it loses energy and becomes reddish.  Therefore, splotchy areas of the CMB map indicate where matter clumped together in the ancient past to form the kernels of the galaxies that we see today. Deeper analysis of the temperature fluctuations may shed light on the galactic evolution process from a very early stage. It gives the timeline of different events occurred in the universe. Study of the temperature fluctuations also throws light on the theory of dark matter and dark energy.
Since 80’s there has been a big quest of detecting the temperature fluctuations on the CMB. The Cosmic Background Explorer in 1989 becomes the first satellite to detect these temperature fluctuations. Since then, several ground-based and satellite-based experiments that measured these fluctuations up to an extraordinarily high precession.  WMAP, BICEP, Planck etc. are some such well-known CMB experiments.
The modern theory about the BigBang cosmology is well consistent with the CMB experiments. However, new anomalies keep on being observed which lead researchers to modify the existing theory and make it more accurate. The WMAP team, in 2010, detected one of such anomaly, known as the statistical isotropy violation. It pose a serious problem to the standard cosmology because it violates one of the fundamental assumption of the BigBand theory, known as the Copernican principle, i.e. at large scale universe looks similar along all the directions. However, findings of WMAP in 2010 challenge this basic assumption of the theory.
There can be two resons behind this phenomenon: firstly, there can be some observational issues, or secondly, the basic assumption of the Copernican principle is in need not true. Several research works to dedicated to study this effect. Finally, very recently a team of cosmologists from IUCAA, India find out the true reason behind this anomaly.
The signal in WMAP satellite was originated due to a phenomenon called instrumental beam. No telescope in this world is ideal. When a telescope looks at a particular direction of the sky, it not only measures the intensity of  light from that particular direction of the sky, but the detected light is contaminated by the lights from the near by directions. If this effect is not accounted for properly in the data analysis technique then it may lead to new unwanted signal in the measured data, which is not present in the original sky.
This was what happened to the measurements by WMAP team. They measured a signal that was not present in the original CMB, and originated just due to the sensitivity of the telescope of WMAP satellite along different directions of the sky. This was first detected by Das et al. (a senior research fellow in IUCAA)  in 2014, with rigorous numerical simulation in supercomputer. Recently Pant et al. (a post-doctorial fellow in IUCAA) device a method to calculate the amount of such spurious signal introduced in different CMB experiments.
No CMB experiment is ideal. Therefore, such misleading signal, however small it may be, will always appear in the CMB experiments. Only thing we can do is to get a prior estimate of the amount of the signal. The research by Pant et al. came on arXiv in 11th November, 2015.

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