Monday, June 13, 2011

SHOWING ITS AGE: The universe might be getting a new birth certificate

By Dr. Sandip Thanki, Ph.D., Associate Professor of Physical Sciences

How old is the Universe?  The answer depends on when you ask the question.  If you had asked the question in the early 1900s, you would have been given an age of 2 billion years – an age that even the astronomers who calculated it did not believe.   They had seen stars older than 10 billion years.  How could the universe be younger than the stars in it?  It’s like saying my toe is older than my body!  In the early 1990s, when some of the youngest NSC students were born, the age of the universe was between 10 and 20 billion years – a range too wide.  A teenager would not be satisfied if her parents told her that her true age is between 10 and 20 years!  
Today, the age of the universe is published as 13.7 billion years in most textbooks. According to a May 2011 pre-print (a scientific journal article to be published), the age of the universe is 12.6 billion years.  So why can’t we finalize the age of the universe?  The simple answer is that we can’t tell how far away things are in the universe.  Let me explain. 
The universe is expanding. Based on how fast it is expanding and how much it has expanded, we can figure out how long it has been expanding for  (that is, we can figure out its age).  If you are driving to Los Angeles from Vegas, I can figure out how long you have been travelling (the “age” of your travel) if you tell me how fast you are going (assuming a constant speed) and how far you have traveled.  For example, if you are moving at a steady 60 mph and have traversed 120 miles, I know that you have been driving for two hours.
A supernova explosion being used as a "Standard Candle"
Image courtesy of High-Z Supernova Search Team, HST, NASA
In the case of the universe, “how fast” is not an issue. It is easy to measure speeds of objects in the sky using Doppler instruments.  This is how we get speeding tickets! The tougher task is to know how far away things are in the universe.  Two major ways to measure distances are through “standard candles” and “standard rulers.” In the case of “standard candles,” when we know how bright something should be and how bright it looks, we can tell how far it is.  This is how we realize how far a car coming towards us is at night because we have a sense of how truly bright the car’s headlights are.  Similarly, we know the brightness of some supernovas (exploding stars).  Based on how bright they look, we can learn how far away they are.  This is the technique that has put the age of the universe at 13.7 billon years.  
One issue with using supernovas as “standard candles” is that it is hard to catch these explosions at great distances.  There also are theoretical scenarios where we do not know the brightness of some supernova explosions.  Even worse, if our understanding of the supernovas changes, the age estimates of the universe will change.  With “standard rulers,” we estimate distances based on how big objects with known sizes look.  For example, when we see the Stratosphere Tower, we can estimate how far we are from it because we have a good sense of how tall it is.  In astronomy, we have not had distant objects with well-known sizes that were sufficiently far away for us to use them as “standard rulers” – until now! 

Cosmic Microwave Backgroudn Radiation.
Image courtesy of Wilkinson Microwave Anisotropy Probe
If you have taken an astronomy class, you might have heard of the cosmic microwave background.  This background is as far as we can see in the universe, and is made of the light that left matter about 400,000 years after the universe was born in the Big Bang.  According to a new theory, Baryon Acoustic Oscillations (BAO), sound waves in the early universe created high-density regions similar to the ones we feel in our ears at the rock concerts, but on a much, much larger scale. When these waves were first created, light and matter were inseparable due to the high temperature and density of the universe. 

Four hundred thousand years after the Big Bang, light separated from matter and the high density regions remained only in form of matter. These regions were the seeds that formed stars and galaxies.  Astronomers have calculated the size of these regions. They are about 490 million light years big and can be our new standard rulers.  One of the most ambitious surveys in astronomy, the Sloan Digital Sky Survey (SDSS), is currently (2008-2014) mapping the universe. The data is public and is released as it becomes available. Stay tuned to find out how old the universe really is.
Scale of large structure pre-determined by Baryon Acoustic Oscillations
Image courtesy of Chris Blake and Sam Moorfield

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