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.
Our universe is full of stars and galaxies everywhere and in all directions.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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...
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)
This article was originally published on Big Think. Read the original article here.
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