- 3314 - UNIVERSE - how much is it expanding? 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. Their light will never get to us. We can not travel faster than light.
--------------------- 3314 - UNIVERSE - how much is it expanding?
- Our universe, everywhere and in all directions, is filled with stars and galaxies. The Milky Way 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.
From our vantage point, we observe up to 46.1 billion light-years away.
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- 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. Our visible universe contains an estimated 2 trillion galaxies.
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- The Hubble eXtreme Deep Field (XDF) may have observed a region of sky just 1/32,000,000 of the total, but was able to uncover 5,500 galaxies within it This is an estimated 10% of the total number of galaxies actually contained in this pencil-beam-style slice.
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- The remaining 90% of galaxies are either too faint or too red or too obscured for Hubble to reveal. However, most of them are already permanently unreachable by us.
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- 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.
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- 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. They most certainly have changed during that time it took their light to reach us.
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- As the universe expands, the space between all unbound objects increases over time.
Beyond distances of 14.5 billion light-years, space’s expansion pushes galaxies away faster than light can travel.
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- Looking back through cosmic time in the Hubble Ultra Deep Field, “ALMA”, traced the presence of carbon monoxide gas. 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.
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- Over time, the expansion rate still drops, but remains positive and large because of “dark energy“. This is the name we give whatever is expanding our universe.
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- The expected fates of the universe 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. Dark energy, whatever that is, is inherent to space itself, never decreases, even as the universe expands.
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- 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 remain constant if it behaves as is currently thought, which is as a “form of energy intrinsic to space itself“.
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- 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. But, our telescopes are getting better.
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- 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.
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- The present “reachability limit” has a boundary 18 billion light-years away. 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. Hard to wrap your mind around this!
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- Beyond about 18 billion light-years, we can never access a galaxy even if we traveled towards it at the speed of light. All galaxies closer than that could be reached if we left today; all galaxies beyond that are unreachable.
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- 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.
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- Only 6% of presently observable galaxies remain reachable; 94% already lie beyond our reach. Each year, another 160 billion stars, enough to compose one major galaxy, become newly unreachable.
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- The final stars, 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.
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- After 100 billion years, even these galaxies will become unreachable to us, even if we were to leave at the speed of light. After that, only our “Local Group of galaxies” will remain within reach.
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- 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. Maybe? As far as we know?
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- October 24, 2021 UNIVERSE - how much is it expanding? 3314
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--------------------- --- Monday, October 25, 2021 ---------------------------
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