The
universe
is space and everything in it.
[1]
[2]
[3]
[4]
It is made of many
billions
of
stars
and
planets
and enormous clouds of gas separated by big spaces. The
Big Bang
started the expansion of the universe.
Astronomers
use
telescopes
to look at distant
galaxies
. This is how they see what the universe looked like a long time ago. The past tense 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
. The size of the universe is not known.
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People have long had ideas about the universe. Most early ideas had the
Earth
at the centre of the universe. This is known as
geocentrism
.
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
.
Over hundreds of years, better observations led to
Copernicus
's
Sun
-centred model, known as
heliocentrism
. 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, better telescopes showed astronomers more about the universe. The Solar System is in a
galaxy
made of billions of stars, which we call the
Milky Way
. 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. By measuring the
redshift
of galaxies, cosmologists soon discovered that the Universe is expanding (see:
Hubble
).
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.
[5]
this would make the universe 13.73 billion years old. Since then, the universe has expanded to be at least 93 billion
light years
, or 8.80
×
10
26
metres
, in
diameter
. It is still expanding right now, and the expansion is getting faster.
Astronomers are not sure what is causing the universe to expand. Because of this, they 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: 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
.
The word
universe
comes from the
Old French
word
univers
, which comes from the
Latin
word
universum
.
[6]
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 theory is 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 are needed 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
).
[7]
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.
Usually the universe is thought to be everything that exists, has existed, and will exist.
[8]
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.
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.
It is possible to form
space-times
, each existing but not able to touch, move, or change (
interact
with each other. The entire collection of these separate space-times is denoted as the
multiverse
.
[9]
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
.
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. Even with most of the visible universe, we cannot interact with it in practice. A relatively simple task, so it might seem, would be to communicate within our galaxy. Even if we knew how to send a message successfully, it would be well over 200,000 years before a reply could come back from the far end of the Milky Way, whose diameter is 100,000
light years
. galaxy. The spatial region which we can see is called the
observable universe
.
The Universe is huge. The matter which can be seen is spread over a space at least 93 billion
light years
across.
[10]
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
.
[11]
As an example, our
Milky Way
Galaxy is roughly 100,000 light years in diameter,
[12]
and our nearest sister galaxy, the
Andromeda Galaxy
, is roughly 2.5 million light years away.
[13]
The observable Universe contains more than 2 trillion (10
12
) galaxies
[14]
and, overall, as many as an estimated
1
×
10
24
stars
[15]
[16]
(more stars than all the
grains of sand
on planet
Earth
).
[17]
Typical galaxies range from dwarf galaxies with as few as ten million (10
7
)
stars
up to giants with one
trillion
[18]
(10
12
) stars, all orbiting the galaxy's center of mass. Thus, a rough estimate from these numbers would suggest there are around one sextillion (10
21
) stars in the observable Universe; though a 2003 study by Australian National University astronomers resulted in a figure of 70 sextillion (7 x 10
22
).
[19]
The matter that can be seen is spread throughout the Universe when averaged over distances longer than 300 million light-years.
[20]
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.
[21]
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 the
cosmic microwave background radiation
.
[22]
[23]
[24]
Independent estimates (based on measurements such as
radioactive dating
) agree, although they are less precise, ranging from 11 to 20 billion years.
[25]
to 13?15 billion years.
[26]
The Universe has not been the same at all times in its history. Its 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.
[27]
The Universe seems to have much more
matter
than
antimatter
.
[28]
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.
[29]
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.
[30]
However, the Universe may have more dimensions, and its spacetime may have a multiply connected global topology.
[31]
As far as we can tell, the Universe has the same
physical laws
and
physical constants
throughout.
[32]
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
.
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
),
[33]
[34]
that is related to an energy density of empty space.
[35]
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,
[36]
recent astronomical observations of type Ia
supernovae
have detected a large amount of
dark energy
that is accelerating the Universe's expansion.
[37]
Preliminary studies suggest that this dark energy is related to a positive
Λ
, although alternative theories cannot be ruled out as yet.
[38]
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.
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. It is not known if the multiverse is a scientific idea.
This is a scientific topic called "the ultimate fate of the universe". It is a topic in
cosmology
. There are possible scenarios for its evolution. The basic issue is whether its existence is finite or infinite.
The future of the universe is a mystery. However, there are a couple of theories based on the possible shapes of the universe:
[39]
- 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 open 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
.
- After a
googol
years, the
black holes
will also be gone. This is the
heat death of the universe
, or Big Freeze theory.
There is a consensus among
cosmologists
that the
shape of the universe
is considered "flat" (
parallel lines
stay parallel) and will continue to expand forever.
[40]
[41]
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Krauss L.M. & Chaboyer B. (2003).
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. Astrophysics (Astronomy Frequently Asked Questions)
. Retrieved
2007-01-04
.
- ↑
Einstein A. (1917). "Kosmologische Betrachtungen zur allgemeinen Relativitatstheorie".
Preussische Akademie der Wissenschaften, Sitzungsberichte
1917 (part 1): 142?152.
- ↑
Rindler (1977), pp. 226?229.
- ↑
Landau and Lifshitz (1975), pp. 358?359.
- ↑
Einstein, A
(1931). "Zum kosmologischen Problem der allgemeinen Relativitatstheorie".
Sitzungsberichte der Preussischen Akademie der Wissenschaften, Physikalisch-mathematische Klasse
.
1931
: 235?237.
Albert Einstein
;
Willem de Sitter
(1932).
"On the relation between the expansion and the mean density of the Universe"
.
Proceedings of the National Academy of Sciences
.
18
(3): 213?214.
Bibcode
:
1932PNAS...18..213E
.
doi
:
10.1073/pnas.18.3.213
.
PMC
1076193
.
PMID
16587663
.
- ↑
Hubble Telescope news release
- ↑
BBC News story: Evidence that dark energy is the cosmological constant
- ↑
Woollaston, Victoria (2016-10-10).
"A big freeze, rip or crunch: how will the universe end?"
.
Wired UK
.
ISSN
1357-0978
. Retrieved
2019-03-15
.
- ↑
"WMAP- Shape of the Universe"
.
map.gsfc.nasa.gov
.
- ↑
"WMAP- Fate of the Universe"
.
map.gsfc.nasa.gov
.
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