17th March 2014 marked a
historic day in cosmology. The team working on the BICEP2 experiment at the
South Pole announced their results, which confirmed detection of the first
direct evidence of the theory of the origin of our universe, the so-called
‘cosmological inflation’. This was immediately followed by shock and awe in the
scientific community worldwide, with the media covering this discovery
extensively. So what exactly is the inflationary model of the universe and why is this discovery so ground breaking? Let’s try and find out!
To understand any model that describes our
universe, it is essential to treat space and time as a unified set of
coordinates, known as ‘spacetime’. Here, time is treated simply as a
coordinate, in addition to (x, y, z) that we already use to describe the three
dimensional space. Thus, spacetime has 4 coordinates, (t, x, y, z). Another key
ingredient that aids in forming models of the universe is the cosmological
principle. This states that the universe appears to be roughly ‘homogenous and isotropic’,
i.e. it appears roughly uniform in any and every direction in space that we
look. Now that the basics are in place, we can proceed to track the models that
describe the evolution of our universe.
In 1929, Edwin Hubble made a landmark
discovery. Using his fairly sophisticated telescope, he observed that all the galaxies seemed
to be moving away from us, and further the galaxy was situated, faster it was receding away from us.
Tracking back this behaviour, one can easily conclude that all the galaxies
must have been very close to each other in the past, quite possibly even
condensed into a single blob of matter, and were suddenly flung outwards. This would result in every galaxy moving away from each other today. This can be understood
by considering the following example: Imagine you have a deflated balloon, and
you mark small dots on its surface with a marker. When you start inflating the
balloon, you would notice that all the dots are receding away from each other.
This is what the ‘big bang theory’ postulated. That all the visible matter was
once condensed into a single entity, and a ‘bang’ resulted in everything that
we see today moving away from each other.
This model, however, had its own problems.
The most pressing one was that the universe looks roughly the same in every
direction that we look. Comparing the distances between the farthest galaxies
in the east direction with the farthest ones in west, we conclude that the
distance between them is too large for light to have travelled from one galaxy
to another (the speed of light being a universal constant) in the known lifetime of our universe (roughly 14 billion years).
Thus, the 2 galaxies could never have been in causal contact! Despite this, why
do they still appear homogenous? This is almost like a situation in which you
encounter an alien flying in from a galaxy far, far away, and that they look
exactly the same as us humans, sharing the same DNA! You would immediately be
tempted to think that our species must have been in contact at some time in the
past to account for the striking similarities. The big bang failed to explain
how these two patches of the sky were causally connected at one point in time.
This is where the inflationary model came to the rescue.
A timeline showing the evolution of our universe. Inflation exponentially expands the space in a very short period of time.
The inflationary model was initially
proposed by Alan Guth in 1980, and further refined by Andrei Linde, Andreas
Albrecht and Paul Steinhardt. Cosmological inflation is the sudden exponential
expansion of space with the expansion rate being greater than the speed of light, when
the universe was just 10-36 seconds old. This inflation continued till about 10-32 seconds, but in this short period of time the universe
had grown drastically in size. The energy scales at which the inflation model
operates determine these time scales. Higher the energy, smaller the time scale
at which it acts. Following the end of inflation, the universe continued to
expand, but at a much smaller rate. A direct consequence of this model was that
parts of the universe could have been connected in the past, and as a result of exponential expansion of space, they appear to never have been in contact when observed today. This beautifully explained why the
universe looks homogenous and isotropic!
Einstein’s General Relativity predicted
that this sudden expansion of the universe must give rise to something known as
‘gravitational waves’. I urge you to look it up, as they are extremely
fascinating. These waves should be neatly imprinted in the
remnant of the big bang that is observable today, the cosmic microwave background
(CMB) radiation.
The BICEP2 experiment remarkably observed
these gravitational waves in the CMB, thus providing the first direct evidence
of cosmological inflation. This is the reason why there has been pandemonium in
the scientific community over the past week, as this strongly supports our
models of the beginning of the universe. This is a huge leap in our understanding of its origin and evolution! Even though their findings look quite
comprehensive at first glance, several experiments all around the world
are rushing to confirm their results. When confirmed by several other experiments,
a Nobel Prize is guaranteed! The only question is, will the theorists who
proposed the model or the team that discovered the signatures receive it.
In either case, this is an amazing leap for human understanding of our universe
and demonstrates the amazing capabilities of current science and technology.
The sun sets behind BICEP2 (in the foreground) and the South Pole Telescope (in the background). (Steffen Richter, Harvard University)
