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| The Hubble
Ultra-Deep Field image shows some of the most
remote galaxies visible with present technology,
each consisting of billions of stars. (Apparent
image area about 1/79 that of a full moon). |
Universe
The Universe is all of time and space and its contents.
It is made of many millions of millions of stars and
planets and enormous clouds of gas separated by a big
space.
Astronomers can use telescopes to look at very distant
galaxies. This is how they see what the Universe looked
like a long time ago. This is because the light from
distant parts of the Universe takes a very long time to
reach us. From these observations, it seems the physical
laws and constants of the Universe have not changed.
Physicists are currently unsure if anything existed
before the Big Bang. They are also unsure whether the
size of the Universe is infinite.
History
People have long had ideas to explain the Universe. Most
early models had the Earth at the centre of the
Universe. Some ancient Greeks thought that the Universe
has infinite space and has existed forever. They thought
it had a set of celestial spheres which corresponded to
the fixed stars, the Sun and various planets. The
spheres circled about a round but unmoving Earth. |
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| Model of the
Copernican Universe by Thomas Digges in 1576,
with the amendment that the stars are no longer
confined to a sphere, but spread uniformly
throughout the space surrounding the planets. |
Over hundreds of years, better observations led to
Copernicus's Sun-centred model. This was very
controversial at the time, and was fought by religious
authorities, most famously by the Christian church (see
Giordano Bruno and Galileo).
The invention of the telescope in the Netherlands, 1608,
was a very important moment in astronomy. By the middle
of the 1800s, telescopes were good enough for other
galaxies to be seen. The modern optical (uses visible
light) telescope is still more advanced. Meanwhile,
Isaac Newton improved the ideas of gravity and dynamics
(equations) and showed how the Solar System worked.
In the 1900s, even better telescopes led astronomers to
realize that the Solar System is in a galaxy made of
billions of stars, which we call the Milky Way. They
also realized that other galaxies exist outside it, as
far as we can see. This started a new kind of astronomy
called cosmology, in which astronomers study what these
galaxies are made of and how they are spread out through
so they can learn more about the history of the Universe
and how it works. By measuring the redshift of galaxies,
cosmologists soon discovered that the Universe is
expanding (see: Hubble). |
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| A theoretical
timeline of the universe. |
Big Bang
The most used scientific model of the Universe is known
as the Big Bang theory, which says the Universe expanded
from a single point that held all the matter and energy
of the Universe. There are many kinds of scientific
evidence that support the Big Bang idea. Astronomers
think that the Big Bang happened about 13.73 billion
years ago, making the Universe 13.73 billion years old.
Since then, the universe has expanded to be at least 93
billion light years, or 8.80 ×1026 metres, in diameter.
It is still expanding right now, and the expansion is
getting faster.
However, astronomers are still not sure what is causing
the universe to expand. Because of this, astronomers
call the mysterious energy causing the expansion dark
energy. By studying the expansion of the Universe,
astronomers have also realized most of the matter in the
Universe may be in a form which cannot be observed by
any scientific equipment we have. This matter has been
named dark matter. Just to be clear, dark matter and
energy have not been observed directly (that is why they
are called 'dark'). However, many astronomers think they
must exist, because many astronomical observations would
be hard to explain if they didn't.
Some parts of the universe are expanding even faster
than the speed of light. This means the light will never
be able to reach us here on Earth, so we will never be
able to see these parts of the universe. We call the
part of the universe we can see the observable universe. |
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| 3rd century BCE
calculations by Aristarchus on the relative
sizes of, from left to right, the Sun, Earth,
and Moon, from a 10th-century AD Greek copy. |
Myths
The word Universe comes from the Old French word Univers,
which comes from the Latin word universum. The Latin
word was used by Cicero and later Latin authors in many
of the same senses as the modern English word is used.
A different interpretation (way to interpret) of
unvorsum is "everything rotated as one" or "everything
rotated by one". This refers to an early Greek model of
the Universe. In that model, all matter was in rotating
spheres centered on the Earth; according to Aristotle,
the rotation of the outermost sphere was responsible for
the motion and change of everything within. It was
natural for the Greeks to assume that the Earth was
stationary and that the heavens rotated about the Earth,
because careful astronomical and physical measurements
(such as the Foucault pendulum) are required to prove
otherwise.
The most common term for "Universe" among the ancient
Greek philosophers from Pythagoras onwards was το παν
(The All), defined as all matter (το ολον) and all space
(το κενον). |
Broadest meaning
The broadest word meaning of the Universe is found in De
divisione naturae by the medieval philosopher Johannes
Scotus Eriugena, who defined it as simply everything:
everything that exists and everything that does not
exist.
Time is not considered in Eriugena's definition; thus,
his definition includes everything that exists, has
existed and will exist, as well as everything that does
not exist, has never existed and will never exist. This
all-embracing definition was not adopted by most later
philosophers, but something similar is in quantum
physics.
Definition as reality
Usually the Universe is thought to be everything that
exists, has existed, and will exist. This definition
says that the Universe is made of two elements: space
and time, together known as space-time or the vacuum;
and matter and different forms of energy and momentum
occupying space-time. The two kinds of elements behave
according to physical laws, in which we describe how the
elements interact.
A similar definition of the term universe is everything
that exists at a single moment of time, such as the
present or the beginning of time, as in the sentence
"The Universe was of size 0".
In Aristotle's book The Physics, Aristotle divided το
παν (everything) into three roughly analogous elements:
matter (the stuff of which the Universe is made), form
(the arrangement of that matter in space) and change
(how matter is created, destroyed or altered in its
properties, and similarly, how form is altered).
Physical laws were the rules governing the properties of
matter, form and their changes. Later philosophers such
as Lucretius, Averroes, Avicenna and Baruch Spinoza
altered or refined these divisions. For example,
Averroes and Spinoza have active principles governing
the Universe which act on passive elements.
Space-time definitions
It is possible to form space-times, each existing but
not able to touch, move, or change (interact with each
other. An easy way to think of this is a group of
separate soap bubbles, in which people living on one
soap bubble cannot interact with those on other soap
bubbles. According to one common terminology, each "soap
bubble" of space-time is denoted as a universe, whereas
our particular space-time is denoted as the Universe,
just as we call our moon the Moon. The entire collection
of these separate space-times is denoted as the
multiverse. In principle, the other unconnected
universes may have different dimensionalities and
topologies of space-time, different forms of matter and
energy, and different physical laws and physical
constants, although such possibilities are speculations.
Observable reality
According to a still-more-restrictive definition, the
Universe is everything within our connected space-time
that could have a chance to interact with us and vice
versa.
According to the general idea of relativity, some
regions of space may never interact with ours even in
the lifetime of the Universe, due to the finite speed of
light and the ongoing expansion of space. For example,
radio messages sent from Earth may never reach some
regions of space, even if the Universe would exist
forever; space may expand faster than light can traverse
it.
It is worth emphasizing that those distant regions of
space are taken to exist and be part of reality as much
as we are; yet we can never interact with them, even in
principle. The spatial region within which we can affect
and be affected is denoted as the observable universe.
Strictly speaking, the observable Universe depends on
the location of the observer. By travelling, an observer
can come into contact with a greater region of
space-time than an observer who remains still, so that
the observable Universe for the former is larger than
for the latter. Nevertheless, even the most rapid
traveler may not be able to interact with all of space.
Typically, the 'observable Universe' means the Universe
seen from our vantage point in the Milky Way Galaxy. |
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Basic data on the Universe
The Universe is huge and possibly infinite in volume.
The matter which can be seen is spread over a space at
least 93 billion light years across. For comparison, the
diameter of a typical galaxy is only 30,000 light-years,
and the typical distance between two neighboring
galaxies is only 3 million light-years. As an example,
our Milky Way Galaxy is roughly 100,000 light years in
diameter, and our nearest sister galaxy, the Andromeda
Galaxy, is located roughly 2.5 million light years away.
The observable Universe contains more than 2 trillion
(1012) galaxies and, overall, as many as an estimated
1×1024 stars (more stars than all the grains of sand on
planet Earth).
Typical galaxies range from dwarf galaxies with as few
as ten million (107) stars up to giants with one
trillion (1012) stars, all orbiting the galaxy's center
of mass. Thus, a very rough estimate from these numbers
would suggest there are around one sextillion (1021)
stars in the observable Universe; though a 2003 study by
Australian National University astronomers resulted in a
figure of 70 sextillion (7 x 1022).
The matter that can be seen is spread throughout the
Universe when averaged over distances longer than 300
million light-years. However, on smaller length-scales,
matter is observed to form 'clumps', many atoms are
condensed into stars, most stars into galaxies, most
galaxies into galaxy groups and clusters and, lastly,
the largest-scale structures such as the Great Wall of
galaxies.
The present overall density of the Universe is very low,
roughly 9.9 × 10−30 grams per cubic centimetre. This
mass-energy appears to consist of 73% dark energy, 23%
cold dark matter and 4% ordinary matter. The density of
atoms is about a single hydrogen atom for every four
cubic meters of volume. The properties of dark energy
and dark matter are not known. Dark matter slows the
expansion of the Universe. Dark energy makes its
expansion faster.
The Universe is old, and changing. The best good guess
of the Universe's age is 13.798±0.037 billion years old,
based on what was seen of the cosmic microwave
background radiation. Independent estimates (based on
measurements such as radioactive dating) agree, although
they are less precise, ranging from 11–20 billion years.
to 13–15 billion years.
The Universe has not been the same at all times in its
history. This getting bigger accounts for how
Earth-bound people can see the light from a galaxy 30
billion light-years away, even if that light has
traveled for only 13 billion years; the very space
between them has expanded. This expansion is consistent
with the observation that the light from distant
galaxies has been redshifted; the photons emitted have
been stretched to longer wavelengths and lower frequency
during their journey. The rate of this spatial expansion
is accelerating, based on studies of Type Ia supernovae
and other data.
The relative amounts of different chemical elements —
especially the lightest atoms such as hydrogen,
deuterium and helium — seem to be identical in all of
the Universe and throughout all of the history of it
that we know of. The Universe seems to have much more
matter than antimatter. The Universe appears to have no
net electric charge. Gravity is the dominant interaction
at cosmological distances. The Universe also seems to
have no net momentum or angular momentum. The absence of
net charge and momentum is expected if the Universe is
finite.
The Universe appears to have a smooth space-time
continuum made of three spatial dimensions and one
temporal (time) dimension. On the average, space is very
nearly flat (close to zero curvature), meaning that
Euclidean geometry is experimentally true with high
accuracy throughout most of the Universe. However, the
Universe may have more dimensions, and its spacetime may
have a multiply connected global topology.
The Universe has the same physical laws and physical
constants throughout. According to the prevailing
Standard Model of physics, all matter is composed of
three generations of leptons and quarks, both of which
are fermions. These elementary particles interact via at
most three fundamental interactions: the electroweak
interaction which includes electromagnetism and the weak
nuclear force; the strong nuclear force described by
quantum chromodynamics; and gravity, which is best
described at present by general relativity.
Special relativity holds in all the Universe in local
space and time. Otherwise, general relativity holds.
There is no explanation for the particular values that
physical constants appear to have throughout our
Universe, such as Planck's constant h or the
gravitational constant G. Several conservation laws have
been identified, such as the conservation of charge,
conservation of momentum, conservation of angular
momentum and conservation of energy. |
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Theoretical models
General theory of relativity
Accurate predictions of the Universe's past and future
require an accurate theory of gravitation. The best
theory available is Albert Einstein's general theory of
relativity, which has passed all experimental tests so
far. However, since rigorous experiments have not been
carried out on cosmological length scales, general
relativity could conceivably be inaccurate.
Nevertheless, its predictions appear to be consistent
with observations, so there is no reason to adopt
another theory.
General relativity provides of a set of ten nonlinear
partial differential equations for the spacetime metric
(Einstein's field equations) that must be solved from
the distribution of mass-energy and momentum throughout
the Universe. Since these are unknown in exact detail,
cosmological models have been based on the cosmological
principle, which states that the Universe is homogeneous
and isotropic. In effect, this principle asserts that
the gravitational effects of the various galaxies making
up the Universe are equivalent to those of a fine dust
distributed uniformly throughout the Universe with the
same average density. The assumption of a uniform dust
makes it easy to solve Einstein's field equations and
predict the past and future of the Universe on
cosmological time scales.
Einstein's field equations include a cosmological
constant (Lamda: Λ), that is related to an energy
density of empty space. Depending on its sign, the
cosmological constant can either slow (negative Λ) or
accelerate (positive Λ) the expansion of the Universe.
Although many scientists, including Einstein, had
speculated that Λ was zero, recent astronomical
observations of type Ia supernovae have detected a large
amount of dark energy that is accelerating the
Universe's expansion. Preliminary studies suggest that
this dark energy is related to a positive Λ, although
alternative theories cannot be ruled out as yet.
Big Bang model
The prevailing Big Bang model accounts for many of the
experimental observations described above, such as the
correlation of distance and redshift of galaxies, the
universal ratio of hydrogen:helium atoms, and the
ubiquitous, isotropic microwave radiation background. As
noted above, the redshift arises from the metric
expansion of space; as the space itself expands, the
wavelength of a photon traveling through space likewise
increases, decreasing its energy. The longer a photon
has been traveling, the more expansion it has undergone;
hence, older photons from more distant galaxies are the
most red-shifted. Determining the correlation between
distance and redshift is an important problem in
experimental physical cosmology.
Other experimental observations can be explained by
combining the overall expansion of space with nuclear
physics and atomic physics. As the Universe expands, the
energy density of the electromagnetic radiation
decreases more quickly than does that of matter, since
the energy of a photon decreases with its wavelength.
Thus, although the energy density of the Universe is now
dominated by matter, it was once dominated by radiation;
poetically speaking, all was light. As the Universe
expanded, its energy density decreased and it became
cooler; as it did so, the elementary particles of matter
could associate stably into ever larger combinations.
Thus, in the early part of the matter-dominated era,
stable protons and neutrons formed, which then
associated into atomic nuclei. At this stage, the matter
in the Universe was mainly a hot, dense plasma of
negative electrons, neutral neutrinos and positive
nuclei. Nuclear reactions among the nuclei led to the
present abundances of the lighter nuclei, particularly
hydrogen, deuterium, and helium. Eventually, the
electrons and nuclei combined to form stable atoms,
which are transparent to most wavelengths of radiation;
at this point, the radiation decoupled from the matter,
forming the ubiquitous, isotropic background of
microwave radiation observed today.
Other observations are not clearly answered by known
physics. According to the prevailing theory, a slight
imbalance of matter over antimatter was present in the
Universe's creation, or developed very shortly
thereafter. Although the matter and antimatter mostly
annihilated one another, producing photons, a small
residue of matter survived, giving the present
matter-dominated Universe.
Several lines of evidence also suggest that a rapid
cosmic inflation of the Universe occurred very early in
its history (roughly 10−35 seconds after its creation).
Recent observations also suggest that the cosmological
constant (Λ) is not zero, and that the net mass-energy
content of the Universe is dominated by a dark energy
and dark matter that have not been characterized
scientifically. They differ in their gravitational
effects. Dark matter gravitates as ordinary matter does,
and thus slows the expansion of the Universe; by
contrast, dark energy serves to accelerate the
Universe's expansion.
Multiverse hypothesis
Some people think that there is more than one universe.
They think that there is a set of universes called the
multiverse. By definition, there is no way for anything
in one universe to affect something in another. The
multiverse is not yet a scientific idea because there is
no way to test it. An idea that cannot be tested or is
not based on logic is not science. So it is not known if
the multiverse is a scientific idea.
Future
The future of the Universe is a mystery. However, there
has a couple of theories based on the possible shapes of
the Universe: |
- If the Universe is a closed sphere,
it will stop expanding. The Universe will do the
opposite of that and become a singularity for another
Big Bang. This is the Big Crunch or Big Bounce theory.
- If the Universe is an opened sphere,
it will speed up the expansion. After 22,000,000,000 (22
billion) years, the Universe will rip apart with the
force. This is the Big Rip theory.
- If the Universe is flat, it will
expand forever. All stars will lose their energy for
that and become a dwarf star. After a googol year, the
black holes will also be gone. This is the Heat Death or
Big Freeze theory.
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