Our universe, everywhere and in all directions, is filled with stars and galaxies.
The Milky Way, as seen at La Silla observatory, is a stunning, awe-inspiring sight to anyone, and offers a spectacular view of a great many stars in our galaxy. Beyond our galaxy, however, are trillions of others, nearly all of which are expanding away from us. (Credit: ESO / Håkon Dahle)
From our vantage point, we observe up to 46.1 billion light-years away.
As long as the light from any galaxy that was emitted at the start of the hot Big Bang 13.8 billion years ago would have reached us by today, that object is within our presently observable universe. However, not every observable object is reachable. (Credit: 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) may have observed a region of sky just 1/32,000,000th of the total, but was able to uncover a whopping 5,500 galaxies within it: an estimated 10% of the total number of galaxies actually contained in this pencil-beam-style slice. The remaining 90% of galaxies are either too faint or too red or too obscured for Hubble to reveal. (Credit: HUDF09 and HUDF12 teams; Processing: E. Siegel)
However, most of them are already permanently unreachable by us.
Although there are magnified, ultra-distant, very red and even infrared galaxies in the extreme Deep Field, there are galaxies that are even more distant out there than what we’ve discovered in our deepest-to-date views. These galaxies will always remain visible to us, but we will never see them as they are today: 13.8 billion years after the Big Bang. (Credit: NASA, ESA, R. Bouwens and G. Illingsworth (UC, Santa Cruz))
As the universe expands, the space between all unbound objects increases over time.
This simplified animation shows how light redshifts and how distances between unbound objects change over time in the expanding universe. Note that the objects start off closer than the amount of time it takes light to travel between them, the light redshifts due to the expansion of space, and the two galaxies wind up much farther apart than the light-travel path taken by the photon exchanged between them. (Credit: Rob Knop.)
Beyond distances of ~14.5 billion light-years, space’s expansion pushes galaxies away faster than light can travel.
Looking back through cosmic time in the Hubble Ultra Deep Field, ALMA traced the presence of carbon monoxide gas. This enabled astronomers to create a 3-D image of the star-forming potential of the cosmos. Gas-rich galaxies are shown in orange. You can clearly see, based on this image, how ALMA can spot features in galaxies that Hubble cannot, and how galaxies that may be entirely invisible to Hubble could be seen by ALMA. All of these galaxies will always be visible to us, but not reachable by us. (Credit: B. Saxton (NRAO/AUI/NSF); ALMA (ESO/NAOJ/NRAO); NASA/ESA Hubble)
Over time, the expansion rate still drops, but remains positive and large because of dark energy.
The expected fates of the universe (top three illustrations) all correspond to a universe where the matter and energy combined fight against the initial expansion rate. In our observed universe, a cosmic acceleration is caused by some type of dark energy, which is hitherto unexplained. All of these universes are governed by the Friedmann equations, which relate the expansion of the universe to the various types of matter and energy present within it. (Credit: E. Siegel/Beyond the Galaxy)
Dark energy, inherent to space itself, never decreases, even as the universe expands.
How matter (top), radiation (middle), and a cosmological constant (bottom) all evolve with time in an expanding universe. As the universe expands, the matter density dilutes, but the radiation also becomes cooler as its wavelengths get stretched to longer, less energetic states. Dark energy’s density, on the other hand, will truly remain constant if it behaves as is currently thought: as a form of energy intrinsic to space itself. (Credit: E. Siegel/Beyond The Galaxy)
All galaxies beyond a certain distance always remain unreachable, even at the speed of light.
Our deepest galaxy surveys can reveal objects tens of billions of light years away, but there are more galaxies within the observable universe we still have yet to reveal. There are parts of the universe that are not yet visible today that will someday become observable to us, and there are parts that are visible to us that are no longer reachable by us, even if we traveled at the speed of light. (Credit: Sloan Digital Sky Survey).
The present “reachability limit” has a boundary ~18 billion light-years away.
The size of our visible universe (yellow), along with the amount we can reach (magenta). The limit of the visible universe is 46.1 billion light-years, as that’s the limit of how far away an object that emitted light that would just be reaching us today would be after expanding away from us for 13.8 billion years. However, beyond about 18 billion light-years, we can never access a galaxy even if we traveled towards it at the speed of light. (Credit: Andrew Z. Colvin and Frederic Michel, Wikimedia Commons; Annotations: E. Siegel)
All galaxies closer than that could be reached if we left today; all galaxies beyond that are unreachable.
Given enough time, light that was emitted by a distant object will arrive at our eyes, even in an expanding universe. However, if a distant galaxy’s recession speed reaches and remains above the speed of light, we can never reach it, even if we can receive light from its distant past. (Credit: Larry McNish/RASC Calgary)
Only 6% of presently observable galaxies remain reachable; 94% already lie beyond our reach.
The GOODS-North survey, shown here, contains some of the most distant galaxies ever observed, a great many of which are already unreachable by us. As time marches forward, more and more galaxies suffer this same fate, disconnecting from us. (Credit: NASA, ESA, and Z. Levay)
Each year, another ~160 billion stars — enough to compose one major galaxy — become newly unreachable.
The final ones, in the M81 group, will become unreachable after another ~100 billion years.
Located a mere 3.6 Megaparsecs away from our Local Group, the M81 group is the nearest substantial group of galaxies to our own Local Group, but will remain gravitationally unbound. After ~100 billion years, even these galaxies will become unreachable to us, even if we were to leave at the speed of light. (Credit: Dominique Dierick/flickr)
After that, only our Local Group will remain within reach.
The Local Group of galaxies is dominated by Andromeda and the Milky Way, and additionally consists of about 60 other, smaller galaxies. All are located within ~5 million light-years of one another, with the nearest galactic groups beyond our own remaining gravitationally unbound from ourselves for all-time. (Credit: Antonio Ciccolella/Wikimedia Commons/cca-sa-4.0)
Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words. Talk less; smile more.
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