Today I am going to discuss about one of the
biggest blunder made by Einstein in his field
equation of general relativity. It is too
closely related to today's one of the biggest
cosmological topics Dark energy and quintessence.
As some of you can probably guess, it is about
cosmological constant. It was first time added
by Einstein working in an equation, as an
extra term, to describe the universe in the
situation when it is "static", that is, the
universe is not expanding. On this concept
Olber's paradox can be explained which argues
about why we see the night sky dark even we
are having infinite number of stars? I have
already made a video on this, if u want to
know more about this click on the i card.
But on 1929 Hubble gaves one of his greatest
law which is also the first evidence of big
bang that the universe is expanding and this
made a serious problem for Einstein to work
on his equations. So on 1931 he finally decided
to abandon his concept. But what was the need
that Einstein had try to use the cosmological
constants? Actually he was trying to solve
the differential equations that describe space-time
thus Einstein needed to add a constant term
to avoid having the universe collapse upon
itself. This term balances the attractive
force of gravity contributed by all of the
matter in the universe. Physically, this term
represents the "vacuum energy", the possibility
that empty space possesses density and pressure,
thus preventing the universe from collapsing.
If the universe expands slightly, then the
expansion releases vacuum energy, which causes
yet more expansion. At last Einstein corrected
his equation by simply putting the value of
cosmological constant as zero. But in the
year 1998, it was discovered that the expansion
of universe is accelerating, this implies
us the possibility of positive non-zero value
for the cosmological constant. So we understand
that it's values may positive or zero but
how can we determine it. One of the most preferential
way is by using cosmic microwave background
or CMB. Through the Plank's satellite we measure
the small temperature and density fluctuations
in the early universe between different locations
and the subsequent changes in the radiation
streaming from those locations. From fitting
the CMB’s power spectrum with the parameters
that determine the expansion of the universe,
hence physicists get a value for the cosmological
constant. Another way to determine the cosmological
constant is to deduce the expansion of the
universe from the redshift of the light from
distant sources. According to Quantum field
theory, this space is not empty it is full
of virtual particles which exhibit fluctuations
and it is a collection of quantum fields.
One practical example of this is gravitational
waves because when these waves travel they
must require a medium like sound waves. So
this indicates that their should be something
in the empty space which is providing the
waves as a source. These Quantum field exhibit
fluctuations at their ground state called
as zero point energy. If you want to know
more about zero point energy, I have already
made a video on this U can check it out in
the i card. These zero point fluctuation made
a major role in cosmological constant, but
theoretically when its value is found it gives
enormous rise to vaccum energy. The difference
between theorized vacuum energy from QFT and
observed vacuum energy from cosmology is a
source of major contention, with the values
predicted exceeding observation by some 120 orders
of magnitude, a discrepancy that has been
called the worst theoretical prediction in
the history of physics. This issue is called
the cosmological constant problem and it
is one of the greatest unsolved mysteries
in science. The cosmological constant Λ is
the simplest possible explanation for dark
energy, and is used in the current standard
model of cosmology known as the ΛCDM model.
The cosmological constant also give evidence
for the existence of "multiverse". It was
given by a nobel prize winner scientist Steven
Weinberg, who explained this theory with help
of anthropic principle which deals with the
fact that, why humans came into existence
in this universe. As I told u The Cosmoloigical
Constant is 10^120 times smaller than what
it should be, so the theory of Nature should
have at least 10^120 different values of the
Cosmological Constant. But each universe have
only one value of Cosmoloigical constant.
So it proves that there are many more Universes
other than our Universe. At last it is the
most controversial topic among the physicist,
there are many ways to find its value but
none of them are agreed by any science community.
The widely accepted value is zero. But we
can hope some suitable theory would be given
in the future which can finally explain all
these doubts. So guys that's all for today.
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