Dark energy, a new and more precise measurement

We don’t know what it isdark energy, nor what it is made of, nor how it behaves. To tell the truth, we don’t even know if it exists; However, let’s assume that it makes up about 70% of the observable universe, which gives us a measure of how much more we do not know how nature works. We call it – or rather, scientists call it – dark energy, where the adjective is dark it is a romantic euphemism that is translated precisely as Unknown. The question spontaneously arises: Why do we assume the existence of an entity about which we know nothing and for whose existence there is (almost) no proof? The answer is that dark energy is currently the only entity that can justify experimental observations, especially theAccelerating expansion of the universe, discovery for which Saul Permuter, Brian Smith And Adam G Riess they have won Nobel Prize in Physics 2011 (It should also be said that there are also those who question the fact that the Universe is expanding at an accelerated rate and that this observation is based only on an illusion related to the relative motion of the Earth with respect to the rest of the cosmos.) or that it depends on still unknown gravitational phenomena, but that is another story: the currently most accepted hypothesis remains the one that brings dark energy into play. The fact is that the scientific community has been trying to shed light on so much darkness for some time, and one of the most promising attempts in this sense is the Dark Energy Survey (Des), a large research project with the ambitious goal “Exploring 14 billion years of cosmic history”that is, to go back in time to Big Bang and understand what happened After. The DES is an international consortium of scientific institutions that were founded in the last ten years (more precisely since August 31, 2013). mapped hundreds of millions of galaxies and thousands of supernovae try to find one “common cosmic structure” in which the effects of dark energy could be hidden.

Beyond the theory of relativity

To understand this better, we need to go back general theory of relativity formulated by Albert Einsteinhave been successfully subjected to hundreds of experimental tests so far: according to this theory the expansion of the universe due to the Big Bang should slowly slow down due to the effect gravity; However, observation of the behavior of the most distant supernovae in 1998 showed that the expansion of the universe is actually still accelerating. To justify this observation Cosmologists have formulated several hypotheses: Among these, the two most recognized predict that dark energy exists, as we said, or that the general theory of relativity must be replaced by another, more comprehensive theory that takes into account still unknown elements. This is where experiments like Des come into play, which are specifically designed to understand the reason for the expansion of the universe: the researchers who work on it – over 400 scientists from 25 different institutions in the United States, Spain, Great Britain, Brazil, Germany , Switzerland and Australia use a 570-megapixel digital camera, it is said DECammounted on the Blanco al telescope Cerro Tololo Inter-American Observatoryin the Chilean Andes to search the skies for useful clues to solve the mystery.

Observations upon observations

In the first six years of the project, from 2013 to 2019, the researchers compiled 758 nights of observation in the cosmos to obtain information about over 300 million galaxies That’s billions of light years away from Earth, a total of about 5,000 square degrees (a unit of measurement of the celestial vault) of the southern sky. The observations were then continued and the latest results were recently introduced 234th Congress of American Astronomical Society, just packaged in New Orleans. To comment on them, in the magazine The conversationWas Robert NicholsScientists at University of Surrey and member of the Des collaboration. “The results we presented in New Orleans – writes the researcher – They could bring us closer to a deeper understanding of dark energy. Among other things, they give us the opportunity to check the soundness of the so-called cosmological constanta quantity introduced by Albert Einstein in 1917 for ‘counterweight’ the effects of gravity in the equations of general relativity, obtaining a model of a static universe that neither expands nor contracts.. The cosmological constant has a long and storied history: Einstein himself defined it after he introduced it “my biggest mistake” and removed it in 1929 according to the astronomer’s observations Edwin Hubble They showed that galaxies were becoming increasingly distant from each other and that the universe was therefore anything but static. Over sixty years later, when, as we have seen, it became clear that the universe was not only expanding but also doing so at an accelerated rate, the cosmological constant was partial rehabilitated and associated with an undetectable form of energy with an anti-gravity effect – something that would actually have a lot to do with dark energy.

How much is it worth? w?

“The results of Des – continues Nichol – represent one of the best measures of an extremely elusive parameter, called ‘w’related to the so-called State equation from dark energy. Since the first hypothesis of dark energy, the value of its equation of state has been a question of fundamental importance: More precisely, this state describes the relationship between pressure and energy density. Everything in the universe has an equation of state: determining its value is the first step to understanding the true nature of dark energy. The currently most accepted theory predicts this w = -1 for dark energy, that is, dark energy is the proposed cosmological constant (and then abolished) by Einstein”. Himself w were really equal to -1, then that would mean it As the density of dark energy increases, its pressure increases Negative. “The greater the density of dark energy in the universe, the greater the repulsion force between matter, that is, the greater the tendency for matter to move away from other matter.” And that is exactly what leads to a model of the universe in continuously accelerated expansion. “.

We have mentioned this repeatedly Supernovae as an object of particular interest to cosmologists studying dark energy. These objects, especially the so-called Type Ia supernovaeare the remnants of giant stellar explosions and can be used as if they were a type of stellar explosion “Meter” for the universe, This allows us to measure enormous distances with excellent precision and indirectly also the value of w. And here we come to the most interesting part: “The new techniques available to us and the expansion of the sample studied – says the scientist again – They allowed us to measure the value of w with higher precision compared to what has been done so far. And we measured a value of -0.8. In short, very close to that of -1, but not exactly the same. So dark energy is not the cosmological constant? It’s another thing? And what? So what is the cosmological constant? One moment. “The uncertainty about the measurement of w It is still large enough that there is a 5% chance that the magnitude will be -1. And that’s why we can’t get out of balance with either option.”.

The fate of the universe

The question is not trivial: why of the value of wthe fate of the universe depends on these decimals: “The measure of w could lead to the exclusion of so-called models Big crack, those where the equation of state has a value less than -1 and which predict that the universe will continue to expand at an ever-increasing rate indefinitely until it completely separates galaxies, planetary systems, and even spacetime itself.. What a relief.

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