Dark Energy and Dark Matter: The Invisible Side of the Universe
Cosmologists have worked to measure the deceleration of the Universe's expansion. According to all prevailing theories at the time, after an initial explosion known as the Big Bang, the Universe should have placidly calmed down under the persistent action of gravitational force. The scenarios presented to cosmologists could therefore be reduced to three fundamental scenarios, dependent on three types of deceleration of universe expansion:
- Rapid deceleration (closed universe): indicating a strong presence of mass in the universe that would lead the universe to collapse on itself in a remote future
- Slow deceleration (open universe): sign of a mass presence too weak to brake the universe's expansion
- Border deceleration (flat universe): a mass presence at the limit between the two previous cases
In no case would they have expected the result that Reiss and Perlmutter publicly presented in 1998, namely that the Universe was not only expanding, but was accelerating its own expansion. "Like a driver just as the green light flashes..." Adam Reiss said lapidarilly in a BBC interview. But what does galaxy acceleration mean and why does it deserve such general attention?
Galaxy acceleration, as Michael Turner, the creator of the term dark energy, pointed out, could only be explained in two fundamental ways, each of which would lead to a radical revision of cosmological concepts: either General Relativity theory was profoundly wrong at large distances, or the universe was largely composed of totally unknown energy except for its antigravitational effect. Either abandon General Relativity in its known form, or admit the existence of a completely unknown new type of energy.
Dark Energy
Wanting to save the fundamental principles of General Relativity, cosmologists thus found themselves having to confront an unknown form of energy radically different from everything physically known and called dark energy. About 74% of the energy existing in the universe would therefore be formed by this dark energy, that is, a form of energy absolutely unidentified except by its antigravitational action. While forms of energy such as matter and radiation aim, in fact, to slow the expansion of the universe due to their gravitational attraction, this dark energy would act in the opposite direction, pushing expansion toward acceleration.
Dark Matter
That of dark energy, however, was not the only unknown variable in the equation. Already many years before, another invisible source of energy, not to be confused with the previous one and called dark matter, had been introduced to safeguard the motion of Galaxies. Dark matter, in fact, is a hypothetical form of matter that, not interacting with the electromagnetic field, cannot be seen except thanks to its gravitational effects. This type of matter was first hypothesized by Fritz Zwicky who in 1933, observing the Coma cluster and pedantically applying the virial theorem, realized an evident quantity of mass missing from the equation. In the Coma cluster, therefore, much less matter was seen than resulted according to the calculated gravitational effects. To solve this problem, Zwicky therefore postulated the existence of invisible matter, thus called dark, responsible for holding galaxies together.
Thus, while in 1933 Fritz Zwicky introduced the idea of dark matter that acted by holding galaxies together, in 1998 the existence of another form of energy became evident, called dark energy, responsible for their expansion. The two concepts added together had to account for about 96% of the energy present in the universe. The fate of the Universe thus seemed to appear in the balance between two unknown and antagonistic energy sources, like two universal pulsations: dark matter acting by slowing universal expansion and dark energy acting as a universal accelerant. So at the beginning of the third millennium, cosmologists around the world, until recently convinced they had to fix only a few details in an already outlined scenario, found themselves with their noses out of joint, being able to account for only 4% of the existing universe.
The Cosmological Constant
For a first approximation, every problem can always be solved by adding an appropriate constant. It was thus that the first place where a solution to the nascent cosmological problem was sought was in the addition of the usual cosmological constant. That of the cosmological constant was, in effect, a ballet already known for some time. Born in 1917 as a patch to obtain a universe more conforming to the requirements of the time, the cosmological constant was nothing more than a term introduced in a makeshift way by Einstein in his equations. Einstein's equations, in fact, predicted a dynamic universe, that is, a universe capable of expanding and contracting, which greatly disturbed Einstein for numerous philosophical and epistemological reasons. To solve these problems of a purely philosophical nature, Einstein introduced an ad hoc term that would balance, with an antigravitational effect, the global gravitational action, thus ultimately obtaining a static universe. With Hubble's discovery of galaxy expansion, the discovery of a dynamic and expanding universe, the rationale of the cosmological constant disappeared, it was removed and with it the related discussions ceased. A few years later, however, the cosmological constant was destined to come back into vogue. Sir Arthur Eddington, in fact, realized that with the universe expansion values announced by Hubble, the Big Bang should have occurred only 2 billion years before. A bit too early, considering that the estimated age of the...