The presence of MACHOs, such as non-radiating neutron stars or white dwarfs and substellar objects such as planets, is invoked to partially explain the rapid rotation of the outer parts of the Milky Way. They look for instances of microlensing, a phenomenon in which the star's light is brightened by the gravitational focusing effect of a foreground MACHO.
On an extragalactic scale, large lensing effects have been observed in which a distant quasar's image is split in two by the gravity of foreground galaxies. The MACHO collaboration recently completed its analysis and concluded that they had detected four microlensing events towards the LMC and more than 40 events towards the galactic bulge - significantly more than had been predicted by the standard model of the central structure of the Milky Way. Because the size and shape of the dark halo that is needed to explain the motion of stars in our galaxy depends upon the structure and amount of luminous matter in the galaxy, these observations are forcing a revision of the standard models.
Many astronomers have clung to the theory that the invisible mass that should make up about 90 percent of the universe is in the form of normal matter, such as stars, planets, asteroids, and quiescent black holes, but so dim or dark that it has eluded detection. However, as searches using increasingly powerful and versatile technologies continue to rule out the simplest explanations, the likelihood increases that the solution lies in exotic hypothetical forms of matter such as weakly interacting massive particles WIMPs that pass unscathed and undetected through planets and people.
One way to observe them is by monitoring the brightness of distant stars. As light rays bend when they pass close to a massive object, light from a distant source may be focused by a closer object to produce a sudden brightening of the distant object.
This effect, known as gravitational lensing , depends on how much matter, both normal and dark, is in a galaxy — we can use it to calculate the amount of matter lurking around. However, we now know it is unlikely that enough of these dark bodies could accumulate to make up the vast amount of dark matter that exists. The Kaluza-Klein theory is built around the existence of an invisible "fifth dimension" curled up in space, in addition to the three spatial dimensions we know height, width, depth , and time.
This theory, a precursor to string theory, predicts the existence of a particle that could be a dark matter particle, which would have the same mass as to protons these make up the atomic nucleus together with neutrons.
This kind of particle could interact both via electromagnetism and gravity. However, as it is curled up in a dimension we can't see, we wouldn't observe it by just by looking at the sky. Luckily, the particle should be is easy to look for in experiments as it should decay into particles we can measure — into neutrinos and photons.
However, powerful particle accelerators like the Large Hadron Collider are yet to detect it. Theories combining general relativity and "supersymmetry" predict the existence of a particle called the gravitino.
Supersymmetry, which is a successful theory explaining a lot of observations in physics, states that all "boson" particles — such as the photon light particle — have a "superpartner", the photino, with a property called "spin" a type of angular momentum that differs by a half-integer. The gravitino would be the superpartner of the hypothetical "graviton", thought to mediate the force of gravitation.
And in some models of supergravity where the gravitino is very light, it could account for dark matter. Explore further. More from Other Physics Topics. Use this form if you have come across a typo, inaccuracy or would like to send an edit request for the content on this page. For general inquiries, please use our contact form.
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Last chance to join our Costa Rica Star Party! Learn about the Moon in a great new book New book chronicles the space program. Dave's Universe Year of Pluto. Groups Why Join? Astronomy Day. Neutron stars are also very dim and they would also be candidates for dark matter. Learn more about how gravity is just a manifestation of the curvature of spacetime. Finally, what about very low-mass stars—stars that are so small that they are almost closer to being planets than to being stars?
We have, in our solar system, gas giant planets like Saturn and Jupiter. These are not stars because they are not heavy enough to burn nuclear fuel inside. You can imagine objects that come from the condensation of gas and dust but weigh a hundred times the mass of Jupiter.
That is not heavy enough to turn on nuclear fuel and start it becoming a star. Such an object is called a brown dwarf. It is an object, which is collapsed gas and dust. People invented a clever nickname for these compact objects. Together, these are all candidates for a certain kind of dark matter. MACHOs are a candidate for the type of ordinary matter that is in a hidden, hard-to-see form, and it would look kind of like dark matter.
Yes, we can. Learn more about the constituents of ordinary matter: nuclei and electrons. White dwarfs and neutron stars are dim. If, say, our galaxy was filled with many, many white dwarfs and neutron stars, they would act much like dark matter and would be hard to see. A lot of mass can be packed into these kinds of objects. The problem is that we only have very specific ways to create white dwarfs and neutron stars. First, you have to make a big star. That star that shines has to eventually give off its nuclear fuel and condense into a white dwarf or a neutron star.
To imagine that the universe is full of white dwarfs and neutron stars, then it would have to be filled with all the matter that has been ejected into interstellar space during the process of forming the white dwarfs and neutron stars.
In the process of condensation, these stars give off a lot of mass. Most of the original mass of that star gets ejected into interstellar space. There is no way that we know of to efficiently take ordinary matter and convert any substantial fraction of it into white dwarfs and neutron stars. How do we know there are not a lot of brown dwarfs in the universe? There could be, but our most reasonable extrapolations say that there are not.
We can figure out how many stars are made as a function of their mass. How many very massive stars are there? How many medium-mass stars are there? How many not-so-massive stars are there? From that, we can extrapolate to how many brown dwarfs there probably are.
We could certainly very well be surprised. After all, if the alternative is new laws of physics, we should be thinking very hard. Learn more about how the expansion of the universe is speeding up rather than slowing down.
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