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Neutrinos vs Dark Matter

Updated on March 24, 2010

Dwarf Galaxy

What is Dark Matter?

The study of dark matter is of high interest to astronomers and physicists studying cosmology. Does dark matter even exist? That seems to be the most important question because it would explain many elements of cosmology that cannot be explained yet. How are neutrinos involved in this quest?

The Problem in Cosmology

Since the turn of the 20th century, physics and astronomy have worked hand in hand to change the view of how the universe works. First came relativity, then quantum mechanics, but tagging right behind these theories have been some amazing astronomical discoveries that have verified the most important aspects of physics, Astronomy followed along so to speak. But they have also changed how physicists saw the cosmos, especially with the observation of the Hubble expansion of the universe; this changed how physics, especially relativity saw cosmology. In this frame astronomy lead the charge.

But after 100 years, physicists and astronomers have puzzled over some important observations. Why do galaxies rotate around the center at the same speed - everywhere? If anything, the outer edges of the galaxies should rotate slower than the center. Butobservations show that is not the behavior of the galaxies.

The problem is that the gravitational force of the galaxy is just not strong enough to make that type of rotation symmetrical andomnipresent. So what is causing the galaxy to behave as it does? The current answer is a combination of dark energy and dark matter.

Origin of the Dark Matter Problem

Going back to 1933, Fritz Zwicky, an astronomer, was studying the motions of far off galaxies. In one study he estimated what the total mass of a cluster of galaxies could be determined by measuring their brightness; a standard approach that was then well known to astronomers. However, when he used a different method, known as the viral theorem to mathematically determine the mass of the same group of galaxies, he found an entirely different account of the size; it was 400 times larger than his original estimate.

This discrepancy in mass between the observed and computed became known as the missing mass problem. And for decades Zwicky's findings were undisturbed, that is, until the 1970's. Scientists looking into the same problems began to realize that only by speculating about large amounts of hidden mass could they explain the observations. Furthermore, scientists also realized that it was through the existence of an unseen mass could they also support theories regarding the structure of the Universe. Finally, today, scientists look for dark matter to explain the gravitational motions of galaxies, as well as to validate the current cosmological theories about the origin and the structure, and fate of the universe.

Fritz Zwicky

Neutrinos

Neutrinos are very common in today's Universe and it is suspected that they may play a role in dark matter. But they are elusive particles with almost no mass. One way to create neutrinos is through radioactive decay like the kind that take place in the Sun. They are also theproduct of nuclear reactors, or through astronomical events when cosmic rays hit atoms.

Neutrinos exist in three forms electron, muon, and tauon neutrinos. They also have an antiparticle associated with them, called anti-neutrinos. Whenever protons change into neutrons, or when the reverse process occurs that is, neutrons become protons, anti-neutrinos are the product. One crucial fact is that neutrinos passing through the Earth originate from the Sun, and the number is staggering. More than 50 trillion solar neutrinos pass through the human body every second!

The Initial Conditions of the Universe

The Big Bang occurred 13.7 billion years ago. Initially, the heat was so intense that all matter existed only in the form of energy. It took time for the Universe to cool down and start to create the conditions where protons, neutrons, electrons and other elementary particles could be formed. The initial conditions of the Universe were so hot and dense at one time that even neutrinos interacted many times. In the early Universe the thermal background of neutrinos was in balance with the thermal background of photons. That is the Cosmic Background Radiation. However, since neutrino interactions are so weak, this thermal balance condition only existed for not more than one second after the Big Bang.

Yet the neutrino background is still present today. There are roughly 56 neutrinos for each type of neutrino per cubic centimeter, totaling 336 neutrinos per cubic centimeter in the Universe. At the same time, the number of photons measured on the Cosmic Background Radiation are slightly more numerous with 411 photons per cubic centimeter.

The number of neutrinos in the Universe is very large, so having even a very small amount neutrino mass can provide seriousconsequences for the Universe.

Source: Neutrinos as Dark Matter


Neutrinos as Dark Matter

Is the the Universe dominated by Cold Dark Matter(CDM) or Hot Dark Matter (HDM)? Astronomers look at how galaxies are distributedthroughout the cosmos. In that case, HDM, which is represented mainly by neutrinos, does not account for the observational patternof galaxies seen today. The galaxies are rough with patches of light and dark, and clusters of mass; in other words they are notsmooth. To understand that, we must go back to the early conditions of the Universe.

Originally after the Big Bang, neutrinos emerged with relativistic velocities, those close to the speed of light. Any fluctuations in matterdensity as a consequence would be smoothed out as they flew out through space. In the early Universe, the neutrino density wasconsiderable, and so most of the matter density could be accounted for by neutrinos. So it was a "smooth" Universe with lots of neutrinos making up the majority of the mass.

Source: Hot Dark Matter

Neutrinos as Dark Matter

Dark Matter - Dark Energy - Composition

The Cooling Off Period

Is the the Universe dominated by Cold Dark Matter(CDM) or Hot Dark Matter (HDM)? Astronomers look at how galaxies are distributedthroughout the cosmos. In that case, HDM, which is represented mainly by neutrinos, does not account for the observational patternof galaxies seen today. The galaxies are rough with patches of light and dark, and clusters of mass; in other words they are notsmooth. To understand that, we must go back to the early conditions of the Universe.

Originally after the Big Bang, neutrinos emerged with relativistic velocities, those close to the speed of light. Any fluctuations in matterdensity as a consequence would be smoothed out as they flew out through space. In the early Universe, the neutrino density wasconsiderable, and so most of the matter density could be accounted for by neutrinos. So it was a "smooth" Universe with lots of neutrinos making up the majority of the mass.

Source: Hot Dark Matter

Neutrinos as Dark Matter

Summary

Galaxies are large mass structures studied because the provide many cosmological features that are useful to physics to explain their own theories. Galaxies do not behave the way they are expected, from a gravitation perspective. It seems that their behavior is due to a large gravitational presence that cannot be explained by current measurements of mass.

So dark matter is used to explain how galaxies are structured. Hot and cold dark matter make up the significantportion of the mass in the Universe. Neutrinos are the "hot" dark matter. They represent, however, a small portion of the mass in the universe, whereas "cold" dark matter may account for over 30%.

Source: Dark Matter

What is Dark Matter

Cosmological Composition

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