Dark Energy: From Einstein To Modern Cosmology
319An outline of modern cosmology:
from Einstein's general relativity to dark energy.
We all believe that space like we know it today, was born by the Big Bang:
the main cosmological theory, the most accepted by the scientific community nowadays.
Big Bang theory describes the evolution of space-time and the universe as well.
However, there's nothing we can argue about the event itself: we imagine it
as an explosi* to trigger imagination and grasp the concept, but the "Big Bang" itself is
defined by physics as a "singularity". A singularity is a zone of space-time where physics is
not the same anymore: like a black hole. It's an inscrutable, unsearchable area, where physics
might be different: we cannot say anything about it. The Big Bang wasn't a huge explo*ion
of matter that moved outside to fill an empty universe: Big Bang is the evolution of spacetime, that has been expanding everywhere, from everywhere.
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Let's retrace the path that led us understand the behaviour at large scale of
the universe.
Before we start, we have to make a leap backward to clarify some concepts that
we are going to use it during our travel.
Have you ever wondered how astronomers can investigate celestial objects located
billion light-years from Earth? We certainly cannot go up there.
We can only analyze the electromagnetic radiation emitted. And this we do with
radiotelescopes and other detectors. In physics e.m. radiation analysis is called spectroscopy.
By means of spectroscopy, we can deduce chemical composition, temperature, density,
mass, luminosity, and even distance of celestial objects.
In particular, during the last century, scientists have learned to understand and to take
advantage of a very useful phenomenon that frequently occurs in spectroscopy: the redshift
effect.
1
I know what you are wondering now: what the heck is this redshift?
It's easy to say: do you know Doppler effect? The redshift is the same for e.m. waves.
Let me immediately be clear.
For sure, you certainly already happened to hear the sound of an ambulance.
While the ambulance is approaching to you, you perceive a higher and higher tone of
the siren, as if the wavelength of its sound decreases.
The opposite happens when the ambulance is receding: the tone becomes
lower and lower as if the wavelength increases. For e.m. waves occurs the same. In fact,
the redshift effect is nothing but the doppler effect of e.m. waves.
Therefore, when an emitting object is moving proceeding to Earth,
its radiation appears more and more shifted to lower wavelengths.
Likewise, when an object is moving away from us, we get its radiation more and more
shifted to higher wavelengths.
Terms redshift and blueshift relate to human perception of longer and shorter
wavelengths as red and blue located on the edges of the visible spectrum with longest and
shortest wavelengths. So, when redshift occurs, we can deduce that the emitting object is
moving away further and further.
So, know we have all concepts and we're ready to start our trip and travel along the
story of our comprehension of the universe until nowadays.
Up to 1915, universe had been regarded as a steady system, as all the observations
suggested. However, when Einstein published general relativity,
the equations of the theory, the famous Einstein's field equations, admitted no static
solutions: in other words, led to a dynamic and unstable universe, not static at all.
Therefore Einstein, added a new term to correct the equations, corresponding to a
repulsive force in order to counterbalance the attractive gravity of matter, that would lead to
collapse the universe on itself. Einstein called this term: cosmological constant clarifying that,
unlike gravity, it was independent of matter
density.
2
Still before, in 1912, evidence of a dynamic universe was found:
The American astronomer Vesto Slipher, in fact, had discovered the redshift of galaxies:
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