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What astronomers want everyone to know about dark matter and dark energy

    Abdulaziz Sobh
    By Abdulaziz Sobh

    Categories: Science


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    If you look at what is often reported in the news, you would be under the impression that dark matter and dark energy are houses of cards waiting to be taken down. Theorists are constantly exploring other options; it can be said that individual galaxies and their satellites favor some modifications of gravity to dark matter; There are great controversies about how fast the Universe expands, and the conclusions we have drawn from the supernova data may need to be altered. Since we have made erroneous assumptions in the past in assuming that the invisible universe contained substances that were simply not there, from ether to phlogiston, is not it a great act of faith to assume that 95% of the universe is a form of invisible and invisible energy of what it is to suppose that there is only one defect in the law of gravity.
    The answer is clear, not absolute, according to almost all astronomers, astrophysicists and cosmologists who study the Universe. This is why.Cosmology is the science of what the Universe is, how it came to be in this way, what is its destiny and what it is made of. Originally, these questions were in the realm of poets, philosophers, and theologians, but the twentieth century brought these questions firmly to the realm of science. When Einstein expounded his theory of General Relativity, one of the first things that were realized is that if you fill the space that the Universe forms with any form of matter or energy, it immediately becomes unstable. If space contains matter and energy, it can expand or contract, but all static solutions are unstable. Once we measured the Hubble expansion of the Universe and discovered the leftover glow of the Big Bang in the form of the Cosmic Microwave Background, cosmology became a quest to measure two numbers: the rate of expansion itself and how that rate changed over time. Measure them, and General Relativity tells you everything you might want to know about the Universe.image
    These two numbers, known as H0 and q0, are called the Hubble parameter and deceleration parameter, respectively. If you take a Universe that is full of things and you start to expand it at a particular rate, you would expect it to have those two main physical phenomena, the gravitational pull, and the initial expansion, fight each other. Depending on how everything turned out, the Universe should follow one of these three paths: The Universe expands fast enough that even with all the matter and energy of the Universe, it can slow down the expansion but never reverse it. In this case, the Universe expands forever.
    The Universe begins to expand rapidly, but there are too much matter and energy. The expansion slows down, stops, reverses, and the Universe finally collapses again.
    Or, perhaps, the Universe, like the third bowl of Goldilocks porridge, is the right one. Maybe the speed of expansion and the number of things in the Universe are perfectly balanced, with the expansion rate as subtle as zero.That last case can only happen if the energy density of the Universe is equal to some perfectly balanced value: the critical density. This is really a beautiful configuration because the equations that are derived from General Relativity are completely deterministic here. Measure how the Universe is expanding today and how it was expanding in the past, and you know exactly what the Universe should do. You can figure out how old the universe has to be, what amount of matter and radiation (and curvature, and anything else) it should contain, and all kinds of interesting information. If we could know those two numbers exactly, H0 and q0, we would know immediately the age of the Universe and also what the Universe is made of. Now, we had some preconceptions when we started on this path. For aesthetically or mathematically detrimental reasons, some people preferred the Universe of reclassification, while others preferred the critical Universe and others preferred the open Universe. In reality, all you can do, if you want to understand the Universe, is to examine it and ask what it is made of. Our laws of physics tell us what rules the Universe rules; the rest is determined by the measure. For a long time, the measurements of the Hubble constant were highly uncertain, but one thing was clear: if the Universe made 100% of normal matter, the Universe turned out to be very young.If the speed of expansion, H0, were fast, like 100 km / s / Mpc, the Universe would have only 6,500 million years. Given that the ages of stars in globular clusters - whatever, some of the oldest stars in the Universe - were at least 12 billion years old (and many cited numbers closer to 14-16 billion), the Universe did not I could be so young While some measurements of H0 were significantly lower, such as 55 km / s / Mpc, that gave a universe of 11 billion and more: even younger than the stars we found in it. On the other hand, as more and more measurements entered the 1970s, 1980s and beyond, it became clear that a Hubble constant abnormally low in the 40s or 50s simply did not align with the data.
    At the same time, we began to measure, with good precision, how abundant were the elements of light in the Universe. Big Bang Nucleosynthesis is the science of how much relative hydrogen, helium-4, helium-3, deuterium, and lithium-7 should be left of the Big Bang. The only parameter that is not derivable from the physical constants in these calculations is the baryon-photon ratio, which indicates the density of normal matter in the Universe. (This is relative to the numerical density of the photons, but that is easily measurable from the Cosmic Microwave Background.) While there was some uncertainty at that time, it became clear very quickly that 100% of the matter could not be " normal, "but only about 10% maximum." There is no way for the laws of physics to be correct and give you a Universe with 100% normal matter.image
    In the early 1990s, this began to align with a series of observations that pointed to pieces of this cosmic puzzle: The oldest stars had to be at least 13 billion years old, If the Universe were made of 100% matter, the value of H0 could not be greater than 50 km / s / Mpc to obtain such an old Universe, Galaxies and galaxy clusters showed strong evidence that there was a lot of dark matter,X-ray observations of the clusters showed that only 10-20% of the matter could be normal matter, The large-scale structure of the Universe (correlations between galaxies in hundreds of millions of light-years scales) showed that it needs more mass than normal matter could provide, but the deep source counts, which depends on the volume of the Universe and how that changes over time, showed that 100% of the matter was too much, Gravitational lenses were beginning to "weigh" these galaxy clusters, and discovered that only about 30% of the critical density was total matter, and the Big Bang Nucleosynthesis really seemed to favor a Universe in which only 1/6 of the density of matter was normal matter. Most astronomers had already accepted dark matter for this time, but even a Universe that was made exclusively of dark and normal matter would still be problematic. I just was not old enough for the stars! Two pieces of evidence in the late 1990s that came together gave us the way forward. One was the information from the supernova, which showed that there was a component in the Universe that caused it to accelerate: this must be dark energy. The other piece of evidence was the cosmic microwave background, which showed us that the Universe was spatially flat, and therefore, the total amount of material there was up to 100%. However, it could not be all matter, even a mixture of dark and normal matter, for a variety of reasons. (Even if you exclude supernova observations!) Looking at the multiple lines of evidence, even today, they all point to that exact image.So, or have all these independent lines of evidence, all pointing to the same image: General Relativity is our theory of gravity, and our Universe is 13.8 billion years old, with ~ 70% dark energy, ~ 30% of total matter, where 5% is normal matter and 25% is dark matter. There are photons and neutrinos that were important in the past, but today they are only a small fraction of a percent. As evidence increases: small-scale fluctuations in the cosmic microwave background, baryonic oscillations in the large-scale structure of the Universe, high-shifting quasars and gamma-ray bursts, this image remains unchanged. Everything that we observe in all the scales indicates it.It was not always apparent that this would be the solution, but this one solution works literally for all observations. When someone hypothesizes that "dark matter and/or dark energy does not exist", the responsibility falls on them to answer the implicit question: "Okay, so what replaces General Relativity is your theory of gravity to explain the whole Universe. "? "As gravitational wave astronomy has further confirmed the great theory of Einstein, even many of the marginal alternatives to General Relativity have disappeared.As it is now, there are no theories that successfully eliminate dark matter and dark energy and keep explaining everything we see, until they exist, there are no real alternatives to the modern image that deserves to be taken seriously, it may not seem right, in your instinct, that 95% of the Universe would be dark. a reasonable possibility when all you have to do, in principle, is to replace your underlying laws with new ones, but until those laws are found, and it has not even been proven that they can exist mathematically, you have to go absolutely with the description of the Universe to which all the evidence points, anything else is simply a non-scientific conclusion.