Dark Matter Constitutes 80% of the Matter in the Universe
|Dark Matter - The Hunt for Dark Matter in the Universe
Dark Matter Constitutes 80% of the Matter in the Universe
For the first time ever, astronomers have been creating a
three-dimensional map of how the dark matter is distributed across the
An international team of scientists, among them groups from
Marseille, the Max-Planck Institutes and Paris have been using data from
the NASA/ESA Hubble Space Telescope.
The results are published in
nature online of January 8th 2007, and at the meeting of the American
Astronomical Society in Seattle. This Video News Release discussed this
In astronomy and cosmology, dark matter is matter that is inferred to exist from gravitational effects on visible matter and background radiation, but is undetectable by emitted or scattered electromagnetic radiation.
Its existence was hypothesized to account for discrepancies between measurements of the mass of galaxies, clusters of galaxies and the entire universe made through dynamical and general relativistic means, and measurements based on the mass of the visible "luminous" matter these objects contain: stars and the gas and dust of the interstellar and intergalactic media.
|According to observations of structures larger than
galaxies, as well as Big Bang cosmology interpreted under the
"Friedmann equations" and the "FLRW metric", dark matter accounts for
23% of the mass-energy density of the observable universe, while the
ordinary matter accounts for only 4.6% (the remainder is attributed to
From these figures, dark matter constitutes 80% of the matter in the universe, while ordinary matter makes up only 20%.
Dark matter was postulated by Fritz Zwicky in 1934, to
account for evidence of "missing mass" in the orbital velocities of
galaxies in clusters.
Subsequently, other observations have indicated the presence of dark
matter in the universe, including the rotational speeds of galaxies,
gravitational lensing of background objects by galaxy clusters such as
the Bullet Cluster, and the temperature distribution of hot gas in
galaxies and clusters of galaxies.
Dark matter plays a central role in state-of-the-art modeling of
structure formation and galaxy evolution, and has measurable effects on
the anisotropies observed in the cosmic microwave background.
All these lines of evidence suggest that galaxies, clusters of galaxies,
and the universe as a whole contain far more matter than that which
interacts with electromagnetic radiation: the remainder is frequently
called the "dark matter component," even though there is a small amount
of baryonic dark matter.
The largest part of dark matter, which does not
interact with electromagnetic radiation, is not only "dark" but also, by
definition, utterly transparent.
The vast majority of the dark matter
in the universe is believed to be nonbaryonic, which means that it
contains no atoms and does not interact with ordinary matter via
The nonbaryonic dark matter includes neutrinos,
and possibly hypothetical entities such as axions, or supersymmetric
baryonic dark matter, nonbaryonic dark matter does not contribute to the
formation of the elements in the early universe ("big bang
nucleosynthesis") and so its presence is revealed only via its
In addition, if the particles of which it is composed are
supersymmetric, they can undergo annihilation interactions with
themselves resulting in observable by-products such as photons and
neutrinos ("indirect detection").
Dark Matter is matter that emits little or no detectable radiation. Gravitational forces observed on many astronomical objects suggest the significant presence of such matter in the universe, accounting for approximately 23 percent of the total mass and energy of the universe.
Its exact nature is not well understood, but it may be largely composed of varieties of subatomic particles that have not yet been discovered, as well as the mass of black holes and of stars too dim to observe. Also called missing mass.
Most Of Our Universe Is Missing
There was a time,
not so long ago, when science seemed to understand how the universe
worked. Everything us, the Earth, the stars and even exotic-sounding
supernovae was made of atoms which were all created at time-zero: the
In between the atoms was nothing, a void: quite literally,
But recently things have started to unravel. There is, it seems, a lot
more to the universe than meets the eye.
According to the best
estimates, we only really know what about 4% of it is made of. But if
only 4% is made of atoms, what about the rest?
The rest is made of
mysterious entities about which very little is understood, with equally
mysterious names: dark matter and dark energy.
The Universe - Dark Matter
Hypothesized Form of Matter Particle that does not
Reflect or Emit Electromagnetic Radiation
Nonbaryonic dark matter is classified in terms of the mass of the particle(s) that is assumed to make it up, and/or the typical velocity dispersion of those particles (since more massive particles move more slowly).
There are three prominent hypotheses on nonbaryonic dark matter, called Hot Dark Matter (HDM), Warm Dark Matter (WDM), and Cold Dark Matter (CDM); some combination of these is also possible.
The most widely discussed models for nonbaryonic dark matter are based on the Cold Dark Matter hypothesis, and the corresponding particle is most commonly assumed to be a neutralino. Hot dark matter might consist of (massive) neutrinos.
Cold dark matter would lead to a "bottom-up" formation of structure in the universe while hot dark matter would result in a "top-down" formation scenario.
In astronomy and cosmology,
dark matter is matter that is inferred to exist from gravitational
effects on visible matter and background radiation, but is undetectable
by emitted or scattered electromagnetic radiation.
Its existence was hypothesized to account for discrepancies between
measurements of the mass of galaxies, clusters of galaxies and the
entire universe made through dynamical and general relativistic means,
and measurements based on the mass of the visible "luminous" matter
these objects contain: stars and the gas and dust of the interstellar
and intergalactic medium.
As important as dark matter is believed to be in the
universe, direct evidence of its existence and a concrete understanding
of its nature have remained elusive.
Though the theory of dark matter remains the most widely accepted theory
to explain the anomalies in observed galactic rotation, some
alternative theories such as modified Newtonian dynamics and
tensor-vector-scalar gravity have been proposed.
None of these alternatives, however, has garnered equally widespread support in the scientific community.
If the dark
matter within our galaxy is made up of Weakly Interacting Massive
Particles (WIMPs), then a large number must pass through the Earth each
second. There are many experiments currently running, or planned, aiming
to test this hypothesis by searching for WIMPs.
Although WIMPs are a more popular dark matter candidate, there are also
experiments searching for other particle candidates such as axions.
is also possible that dark matter consists of very heavy hidden sector
particles which only interact with ordinary matter via gravity.
These experiments can be divided into two classes: direct detection
experiments, which search for the scattering of dark matter particles
off atomic nuclei within a detector; and indirect detection, which look
for the products of WIMP annihilations.
An alternative approach to the detection of WIMPs in nature is to
produce them in the laboratory. Experiments with the Large Hadron
Collider (LHC) may be able to detect WIMPs; because a WIMP has
negligible interactions with matter, it may be detected indirectly as
(large amounts of) missing energy and momentum which escape the LHC
detectors, provided all the other (non-negligible) collision products
These experiments could show that WIMPs can be created, but it would
still require a direct detection experiment to show that they exist in
sufficient numbers in the galaxy, to account for dark matter.
The Missing Universe
What are 'dark matter' and 'dark energy'?
The content of the
Universe is widely thought to consist of three types of substance:
normal matter, dark matter and dark energy.
consists of the atoms that make up stars, planets, human beings and
every other visible object in the Universe.
As humbling as it sounds,
normal matter almost certainly accounts for the smallest proportion of
the Universe, somewhere between 1% and 10%.
The more astronomers
observed the Universe, the more matter they needed to find to explain it
all. This matter could not be made of normal atoms, however, otherwise
there would be more stars and galaxies to be seen. Instead, they coined
the term 'dark matter' for this peculiar substance precisely because it
escapes our detection.
At the same time, physicists trying to
further the understanding of the forces of nature were starting to
believe that new and exotic particles of matter must be abundant in the
These would hardly ever interact with normal matter
and many now believe that these particles are the dark matter. At the
present time, even though many experiments are underway to detect dark
matter particles, none have been successful. Nevertheless, astronomers
still believe that somewhere between 30% and 99% of the Universe may
consist of dark matter.
Dark energy is the latest addition to
the contents of the Universe. Originally, Albert Einstein introduced the
idea of an all-pervading 'cosmic energy'; before he knew that the
Universe is expanding. The expanding Universe did not need a
'cosmological constant' as Einstein had called his energy.
in the 1990s observations of exploding stars in the distant Universe
suggested that the Universe was not just expanding but accelerating as
well. The only way to explain this was to reintroduce Einstein's cosmic
energy in a slightly altered form, called 'dark energy'. No one knows
what the dark energy might be.
In the currently popular
'concordance model' of the Universe, 70% of the cosmos is thought to be
dark energy, 25% dark matter and 5% normal matter.