An 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 universe as well.
However, there's nothing we can argue about the event itself: we imagine it
as an explosion 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 explosion of matter that moved outside to fill an empty
universe: Big Bang is the evolution of spacetime, that has been expanding everywhere, from everyhere.
Let's retrace the path that led us to understand the behavior 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 the 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 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 the 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, the 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 the matter, which
would lead to the collapse of 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: far galaxies were receding.
He was the first to do it.
Slipher used spectroscopy to investigate the rotation periods of planets
and the composition of planetary atmospheres.
The first scientist to seriously apply general relativity to cosmology without the
stabilizing cosmological constant was Alexander Friedmann.
In 1922, Friedmann derived the expanding-universe solution from general relativity
equations, showing that the universe was not static at all, but might be expanding.
Two years later American astronomer Edwin Hubble, combining his own
results of galaxy distances with Slipher's measurements of galactic redshifts,
discovered a rough proportionality of the objects' distances with their
redshifts.
This is known today as Hubble–Lemaître's law was formulated in 1929 and
became the basis for the modern model of the expanding universe: the greater is the distance
between two galaxies and the higher is the speed of their reciprocal distancing.
In 1927, the Belgian priest Georges Lemaitre, in order to explain Slipher's galactic
redshifts, proposed an expanding model for the universe.
Moreover, he suggested in 1931, that in a finite past time, all the mass of the
universe could be concentrated in a single point.
This is a first roughly formulation of the Big
Bang theory of which LeMaitre is considered one of
the fathers.
After these results, that same year, Einstein was convinced about the expansion of the
universe, so definitely abandoned the cosmological constant, defining it as his biggest mistake.
All these observations and contributions led to the development of the first cosmological
model of Big Bang that describes a homogeneous (physics laws are the same everywhere),
isotropic ( no preferred direction in space), expanding universe.
Space-time expansion is modeled following the Friedmann–Lemaître–Robertson–
Walker metric (FLRW).
3 In maths and physics, a "metric" means a measure
of distance, and the term implies that the sense of distance itself, within
the universe, is changing.
It's space itself that expands, very rapidly within the first fraction of a second
after the Big Bang.
This kind of expansion is known as "metric expansion".
To understand this concept let's consider furnishing a room with tables, chairs, cabinet
, and furniture.
Now let's imagine the room expanding, and, little by little, distancing the
furniture and the objects that, while remaining still, are moving away from each other more
and more.
The model is valid at a large scale only (roughly the scale of galaxy clusters and above),
because gravitational attraction binds matter together strongly enough that metric
expansion cannot be observed on a smaller scale.
The model has been refined and enriched over time.
In the late 1940s physicists, Alpher and Herman realized that if the Big Bang theory was
right, expansion of the universe would have necessarily left the footprint of the very
early universe into a microwave region of the electromagnetic
spectrum.
They predict the so-called "cosmic microwave background"(CMB), the "echo"
of Big Bang, but it took another 15 years to detect it.
In 1964, the physicists Arno Penzias and Robert Wilson built a microwave radiometer
that they intended to use for radio astronomy and satellite communication experiments.
That same year, by pure chance, their instrument detected the presence of an
omnidirectional background signal, not associated with any star, galaxy, or another celestial
object.
A faint background noise, or glow, strongest in the microwave region of the radio
spectrum.
The cosmic microwave background (CMB) in Big Bang cosmology, is the oldest e.m.
radiation in the universe, and relates to the time when the light was born, about 380ky
after the Big Bang.
CMB together with the cosmological redshift-distance relation
(Hubble's law) are regarded as the best available evidence for the Big Bang theory.
In 1978, Penzias and Wilson were awarded the Nobel Prize in physics for this discovery.
4 Over time, lots of radiotelescopes and detectors
were put in orbit to measure CMB throughout the universe more and more accurately.
The most famous experiment is probably the NASA Cosmic Background Explorer (COBE)
satellite that orbited in 1989–1996.
In 1992, COBE discovered tiny fluctuations in
the CMB, called anisotropies, little irregularities in the spectrum of the radiation which are
very important to investigate, because could provide precious
information about the very early universe.
COBE detected and quantified the large scale anisotropies at the limit of its
detection capabilities.
COBE was replaced and upgraded by WMAP (Wilkinson Microwave Anisotropy
Probe)to make much more precise measurements of the large scale anisotropies over the full
sky.
Finally, Planck space observatory provided the highest resolution and sensitivity
measurements.
Anisotropy was confirmed also by a series of ground and balloon-based experiments.
All these results, could even assess the geometry of the universe, concluding that
is approximately flat, rather than curved.
----------------------------- In the last decades, we have discovered something
even stranger about the universe.
At large scales, universe has a mysterious behavior and is much more stunning than we
believe.
In 1998 two independent research projects, the Supernova Cosmology Project headed
by Saul Perlmutter and the High-Z Supernova Search Team, headed by Brian Schmidt and
Adam Riess were studying 1a Type supernovae.
Supernovae are such particular stellar explosions, which are so bright that can be
used as a benchmark to deduce the distance of their
host galaxies.
Measuring the redshift of 1a type supernovae scattered in galaxies, scientists
were able to deduce the expanding rate of the universe.
5 Until that moment common opinion was that,
after big bang, the universe was little by little
decelerating, because of the gravitational attraction caused by mass.
The unexpected result was that the universe was increasing its expansion rate.
The 2 projects, as a matter of fact, independently, assessed that the expansion of the
universe was not slowing down at all: it was accelerating.
In 2011, Perlmutter, Schmidt and Riess were awarded the Nobel prize.
------------------------------------------------ In order to justify this acceleration, cosmologists
had to introduce a new type of energy: this is dark energy.
The expression dark energy is to say that the universe, at large scales, seems to
be permeated by a mysterious anti-gravity energy that is prevailing on mass and is causing
the expansion of the universe to accelerate.
We still don't know the nature of this new form of
energy because, as dark matter, we can't detect it directly, we can only get some evidence.
The theory of primordial nucleosynthesis tries to explain the formation of light elements
in the earlier universe like helium, deuterium,
lithium as theory of the large scale structure of the
universe investigates star formations, quasar, galaxies, clusters.
Both theories suggest that if the universe has a flat shape, the energy
density of the whole matter is about only 30% of the total
energy density amount in the universe.
If the universe is flat like demonstrated by anisotropy
CMB analysis, we have to introduce an additional form of energy to account for the
remaining 70%.
According to the best measurements provided by Planck space observatory,
operative since 2009 to 2013, dark energy contributes 68% of the total energy density,
dark matter and ordinary matter contribute 27%
and 5%, respectively, and other components such
as neutrinos and photons contribute a very small amount.
The density of dark energy is very low, much less than the density of ordinary matter
or dark matter within galaxies.
However, it dominates the mass-energy of the universe
because it is uniform across space.
What is dark energy made of?
Why it's stronger than gravity?
Does it interact directly with ordinary matter?
6 We still don't know.
At the moment we can only formulate hypothesis, from the cosmological constant model to the
quintessential model.
1- COSMOLOGICAl CONSTANT Dark energy could be simply a cosmological
constant that permeates all the universe with an antigravitational effect accelerating
its expansion.
This is usually represented by the Greek letter Λ This model, dust off again
the cosmological constant, introduced by Einstein, originally to provide a repulsive force to
counterbalance the attractive gravitation and ensure
a static universe, and afterward abandoned when the expansion of the universe was
assessed.
Cosmological constant had been always supported by Lemaitre to support Big Bang
theory.
Einstein's error was, at least, somewhat reduced: an undetectable form of energy
would actually permeate the space, but its hypothetical antigravity effect, rather than
balancing the universe, would accelerate its expansion.
2- QUINTESSENCE Quintessence differs from the cosmological
constant explanation of dark energy because is dynamic: it can vary in space and time
unlike the cosmological constant which, by definition does not change.
It has been proposed by some physicists to be a fifth fundamental
force.
3-OTHER THEORIES Therefore, it is conceivable that a modification
to general relativity also eliminates the need for dark energy.
There are very many such theories, and research is ongoing...
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