The universe is expanding, with every galaxy beyond the Local Group speeding away from us. Today, most of the universe's galaxies are already receding faster than the speed of light. All galaxies currently beyond 18 billion light-years are forever unreachable by us, no matter how much time passes.
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Our universe is full of stars and galaxies everywhere and in
all directions.
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The Milky Way, as seen at the La Silla Observatory, is a
breathtaking, impressive sight for everyone and offers a spectacular view of
many stars in our galaxy. Beyond our galaxy, however, there are trillions of
others, almost all of which are expanding away from us. (Image: ESO / Håkon
Dahle) |
From our vantage point, we observe up to 46.1 billion light
years away.
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As long as the light of a galaxy, which was emitted at the
beginning of the hot Big Bang 13.8 billion years ago, had reached us today,
this object will be in our currently observable universe. However, not every
observable object can be reached. (Source: F. Summers, A. Pagan, L. Hustak, G.
Bacon, Z. Levay and L. Frattere (STScI)) |
Our visible universe contains an estimated ~ 2 trillion
galaxies.
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The Hubble eXtreme Deep Field (XDF) has observed a region of
the sky of only 1 / 32,000,000 of the total, but was able to discover a
whopping 5,500 galaxies in it: an estimated 10% of the total number of galaxies
actually contained in it. The remaining 90% of the galaxies are either too
faint or too red or too dark for Hubble to reveal. (Source: HUDF09 and HUDF12
teams; processing: E. Siegel) |
However, most of them are already permanently unavailable
for us.
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Although there are magnified, extremely distant, very red
and even infrared galaxies in the eXtreme Deep Field, there are galaxies that
are even further away than we have discovered in our deepest views to date.
These galaxies will always remain visible to us, but we will never see them the
way we do today: 13.8 billion years after the Big Bang. (Image Credit: NASA,
ESA, R. Bouwens and G. Illingsworth (UC, Santa Cruz)) |
As the universe expands, the distance between any unbound
objects increases over time.
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This simplified animation shows how redshifts of light and
how distances between unbound objects in the expanding universe change over
time. Note that the objects start closer than the time it takes for light to
move between them, the light redshifts due to the expansion of space, and the two
galaxies end much further apart than the light path of the photon exchanged
between them. (Image credit: Rob Knop.) |
Beyond distances of ~ 14.5 billion light years, the expansion of space is pushing galaxies away faster than light can travel.
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Looking back through cosmic time in the Hubble Ultra Deep
Field, ALMA tracked the presence of carbon monoxide gas. This enabled
astronomers to create a 3-D image of the star formation potential of the
cosmos. Gas-rich galaxies are shown in orange. Based on this image, you can
clearly see how ALMA can see features in galaxies that Hubble cannot, and how
galaxies that may be completely invisible to Hubble can be seen by ALMA. All of
these galaxies will always be visible to us, but will not be accessible to us.
(Image credit: B. Saxton (NRAO / AUI / NSF); ALMA (ESO / NAOJ / NRAO); NASA /
ESA Hubble) |
Over time, the rate of expansion will still decrease, but it
will remain positive and large due to the dark energy.
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The expected fates of the universe (the top three figures)
all correspond to a universe in which matter and energy fight together against
the initial rate of expansion. In our observed universe, a cosmic acceleration
is caused by some kind of dark energy that has so far been unexplained. All of
these universes are determined by the Friedmann equations, which relate the
expansion of the universe to the various types of matter and energy. (Photo
credit: E. Siegel / Beyond the Galaxy) |
Dark energy, inherent in the space itself, never diminishes
even as the universe expands.
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How matter (above), radiation (middle) and a cosmological
constant (below) develop over time in an expanding universe. As the universe
expands, the density of matter becomes thinner, but the radiation also becomes
cooler as its wavelengths are stretched to longer, less energetic states. The
density of dark energy, on the other hand, will really remain constant if it
behaves as one currently thinks: as an energy form inherent in the room itself.
(Photo credit: E. Siegel / Beyond The Galaxy) |
All galaxies beyond a certain distance always remain
inaccessible, even at the speed of light.
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Our deepest galaxy surveys can reveal objects tens of
billion light years away, but there are more galaxies in the observable
universe that we have yet to uncover. There are parts of the universe that are
not yet visible today and will one day become observable for us, and there are
parts that are visible to us and can no longer be reached by us, even if we are
traveling at the speed of light. (Source: Sloan Digital Sky Survey). |
The current “reachability limit” has removed a limit of ~ 18
billion light years.
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The size of our visible universe (yellow) along with the
amount we can achieve (magenta). The limit of the visible universe is 46.1
billion light years, as this is the limit of how far an object that emits light
that would just reach us today would be after an expansion of 13.8 billion
years. However, beyond about 18 billion light years, we can never access a
galaxy, even if we are heading towards it at the speed of light. (Source:
Andrew Z. Colvin and Frederic Michel, Wikimedia Commons; Notes: E. Siegel) |
Any galaxies that are closer could be reached if we break up
today; all galaxies beyond that are inaccessible.
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With sufficient time, light emitted from a distant object
will reach our eyes even in an expanding universe. However, if the speed of
recession of a distant galaxy reaches the speed of light and stays above it, we
can never reach it even if we can receive light from its distant past. (Photo
credit: Larry McNish / RASC Calgary) |
Only 6% of the currently observable galaxies can still be
reached; 94% are already out of our reach.
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The GOODS North survey shown here contains some of the most
distant galaxies ever observed, many of which are already inaccessible to us.
Over time, more and more galaxies suffer the same fate and separate from us.
(Source: NASA, ESA and Z. Levay) |
Every year another ~ 160 billion stars – enough to form a
large galaxy – become inaccessible.
The last ones in the M81 group will be unreachable after
another ~ 100 billion years.
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The M81 group is only 3.6 megaparsecs away from our local
group and is the galaxy group closest to our own local group, but will remain
gravitationally unbound. After ~ 100 billion years, even these galaxies will
become inaccessible to us, even if we were to disappear at the speed of light.
(Photo credit: Dominique Dierick / Flickr) |
After that, only our local group remains within reach...
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The local galaxy group is dominated by Andromeda and the
Milky Way and also consists of about 60 other, smaller galaxies. All are within
~ 5 billion light years of each other, with the closest galactic groups beyond
our own remaining gravitationally unbound of ourselves for all time. (Photo
credit: Antonio Ciccolella / Wikimedia Commons / cca-sa-4.0) |