Big Bang and the Age of the Universe

If the rate of expansion one second after the Big Bang had been smaller by even one part in hundred thousand million million, the universe would have recollapsed before it ever reached it's present size - Stephen Hawking

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Big Bang and the Age of the Universe
Big Bang and the Age of the Universe (Credit: Dr. Gerald L. Schroeder, The Science of God, The Free Press, NY,NY 1997.)

Astronomers calculated that the Big Bang occurred between twelve and fourteen billion years ago. To place this in perspective, the solar system is assumed to be 4.5 billion years old and humans have existed for a few million years.

Astronomers estimate the age of the universe in 2 ways:

  1. By trying to find the oldest stars
  2. By finding out the speed of expansion of the universe and extrapolating back to the Big Bang

Is it older than the stars?

Astronomers could place a lower limit to the age of the universe by studying about Globular clusters. These circular clusters are a dense assortment of roughly 1,000,000 stars. Stellar densities close to the middle of these clusters are huge. If we lived close to the middle of one, there would be several hundred thousand stars closer to us than the Proxima Centauri, the nearest star after the Sun.

The life cycle of a star is solely depended upon its mass. High Mass stars are a lot brighter than Low Mass stars; therefore they quickly burn through their supply of hydrogen fuel. A star just like the Sun has enough fuel in its core to burn at its current brightness for roughly nine billion years. A star that is twice as massive as the Sun will burn through its fuel supply in only 800 million years. A 10 solar mass star, a star that is 10 times more massive than the Sun, burns nearly a thousand times brighter and has only a 20 million year fuel supply. Conversely, a star that is half as massive as the Sun burns slowly enough for its fuel to last more than 20 billion years.

All the stars in a globular cluster are formed roughly at the same time; therefore they function like cosmic clocks. If a cluster is over twenty million years old, then all of its hydrogen-burning stars will be less massive than 10 solar masses. This suggests that no hydrogen-burning star is going to be over a thousand times brighter than the Sun. If a cluster is over two billion years old, then there’ll be no hydrogen-burning star more massive than 2 solar masses.

The oldest cluster contains only those stars which are less massive than 0.7 solar masses. Low Mass stars are a lot less dim than the Sun. This observation suggests that the oldest clusters are between 11 and 14 billion years old. The uncertainty in this estimate is because of the problem in finding the precise distance to cluster (hence, uncertainty within the brightness (and mass) of the stars in the cluster). Another source of uncertainty during this estimate lies in our ignorance of finer details of stellar evolution.

Astronomers also study some old stars and try to detect remaining dust from ancient supernova to have some more limits to the age of the universe.

However, there are some stars found which contradicts the currently believed age of 13.8 billion years. Read about one such star called HD 140283 here

Hubble constant

An alternative approach to estimate the age of the universe is to calculate the “Hubble constant”. The Hubble constant is a measure of the expansion rate of the universe. Cosmologists use this activity to extrapolate back to the Big Bang. This extrapolation depends on the history of the expansion rate that depends on the present density of the universe and on the composition of the universe.

If the universe is flat and composed largely of matter, then the age of the universe is

2/(3 Ho)

where Ho denotes the Hubble’s constant.

If the universe has a very low density of matter, then its extrapolated age is larger:

1/Ho

If the universe contains a form of matter similar to the cosmological constant, then the inferred age can be even larger.

Cosmic distance ladder

Many astronomers are operating exhaustingly to calculate the Hubble’s constant employing a type of completely different techniques. Till recently, the simplest estimates ranged from 65 km/sec/Megaparsec to 80 km/sec/Megaparsec. In additional acquainted units, astronomers believe that 1/Ho is between 12 and 14 billion years.

The latest news on Hubble constant:

Researchers detect abnormal rates in the expansion of universe

In the latest researches, the results have been shown that the universe is enlarging much quicker than it should be based on the conditions after Big Bang. The constantly increasing rate of the universe is known as Hubble Constant and it has been very hard to pin down the rate.

AN AGE CRISIS?

On comparing the two age determinations, there is a potential crisis. If the universe is flat and dominated by ordinary or dark matter, the age of the universe as estimated from the Hubble constant would be around 9 billion years. This would lead to a contradiction as the age of the universe would be shorter than the age of oldest stars. This contradiction implies that either

  1. Our measurement of the Hubble constant itself is incorrect
  2. The Big Bang theory has to be replaced
  3. That we need a form of matter like a cosmological constant that implies an older age for a given observed expansion rate.

Some astronomers believe that this crisis will pass as soon as measurements improve. If the astronomers who have measured the smaller values of the Hubble constant are correct, and if the smaller estimates of globular cluster ages are also correct, then all is well for the Big Bang theory, even without a cosmological constant.

WMAP satellite

Measurements by the WMAP (Wilkinson Microwave Anisotropy Probe) satellite facilitated and confirmed the age of the universe. By measuring the thermal radiation left over from the Big Bang(known as Cosmic Microwave Background), missions such as WMAP are able to determine the density, composition and expansion rate of the universe. As of 2013, WMAP determined these parameters with an accuracy of higher than 1.5%. In turn, knowing the composition with this precision, we could estimate the age of the universe to about 0.4%: 13.77 ± 0.059 billion years!

WMAP satellite artist depiction from NASA
WMAP satellite artist depiction. Image Source: NASA

How does WMAP data enable us to determine the age of the universe is 13.77 billion years, with an uncertainty of only 0.4%? The key to this is that by knowing the composition of matter and energy density in the universe, we will use Einstein’s theory of relativity to calculate how fast the universe has been expanding in the past. With this information, we will flip the clock back and estimate when the universe had “zero” size, in line with Einstein.

The time between then and now could be the age of the universe. There’s one caveat to keep in mind that affects the understanding of the age determination: we have a tendency to assume that the universe is flat, that is well supported by WMAP and alternative knowledge. If we relax this assumption, the uncertainty will increase. Inflation naturally predicts a really nearly flat universe.

The expansion age measured by WMAP is larger than the oldest globular clusters, so, Big Bang theory has passed a crucial test using data independent of the type collected by WMAP. If the expansion age measured by WMAP had been smaller than the oldest globular clusters, then there would be something wrong in either the Big Bang theory or the theory of stellar evolution. Either way, astronomers would have required rethinking several of their cherished concepts. However, our current estimate of age fits well with what we all know from other forms of measurements.

Final Words

So, in conclusion, we’ve 2 methods — one from our cosmic history and one by measuring native stars — that show us our Universe’s age is between thirteen and fourteen billion years. Today it is widely accepted that our universe in 13.772 billion years old with a possible error of 59 million years. However, there are a lot of observations and experiments going on to get more precise age of our universe

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