Do we observe galaxies disappearing at the edge of the observable universe?

[am77494]

@levdrakon

This gives the observable universe a boundary, which presently is thought to be 16 billion light years away.

The universe could be infinite. …
IANAA.

Do the objects that have gone beyond the boundary continue to exert forces like gravity ? (like a black hole does ?). If not, then for all practical aspects they do not exist - right ? If they are not observable or otherwise detectable - they do not exist seems to be the notion of science - am I misunderstanding this ?

[levdrakon]

@eburacum45

Sorry to be a naysayer, but this isn’t right either.

There are lots of different ways of measuring the distance of the edge of the visible universe, but none of them give the distance as 16 billion light years.
See this wikipedia page;
Observable universe - Wikipedia

Yeah, I shouldn’t post about this stuff at night when I’m tired. The 16 billion ly figure I used was referring to a cosmic event horizon, mentioned in the same article you linked to, just above it in the The universe versus the observable universe section:

Though in principle more galaxies will become observable in the future, in practice an increasing number of galaxies will become extremely redshifted due to ongoing expansion, so much so that they will seem to disappear from view and become invisible.[7][8][9] An additional subtlety is that a galaxy at a given comoving distance is defined to lie within the “observable universe” if we can receive signals emitted by the galaxy at any age in its past history (say, a signal sent from the galaxy only 500 million years after the Big Bang), but because of the universe’s expansion, there may be some later age at which a signal sent from the same galaxy can never reach us at any point in the infinite future (so for example we might never see what the galaxy looked like 10 billion years after the Big Bang),[10] even though it remains at the same comoving distance (comoving distance is defined to be constant with time—unlike proper distance, which is used to define recession velocity due to the expansion of space), which is less than the comoving radius of the observable universe. This fact can be used to define a type of cosmic event horizon whose distance from us changes over time. For example, the current distance to this horizon is about 16 billion light years, meaning that a signal from an event happening at present can eventually reach us in the future if the event is less than 16 billion light years away, but the signal will never reach us if the event is more than 16 billion light years away.[5]

[levdrakon]

@am77494

Do the objects that have gone beyond the boundary continue to exert forces like gravity ? (like a black hole does ?). If not, then for all practical aspects they do not exist - right ? If they are not observable or otherwise detectable - they do not exist seems to be the notion of science - am I misunderstanding this ?

Yes they do, but the effects of gravity are thought to propagate at c, so we won’t ever directly detect them if they originate too far away. Gravity has a lot to do with how the universe is arranged and put together, so we can make guesses as to how much is out there. The universe is thought to be homogeneous and isotropic, so we can assume our part of the universe is more or less like every other part of the universe.

Hopefully one of my favorite posters like Stranger on a Train will show up and spread some knowledge around.

[eburacum45]

@levdrakon

Yeah, I shouldn’t post about this stuff at night when I’m tired. The 16 billion ly figure I used was referring to a cosmic event horizon, mentioned in the same article you linked to, just above it in the The universe versus the observable universe section:

That is actually a really interesting value; thanks for pointing it out!

[colonial]

@The_Hamster_King

It is correct.

It’s true that the source of the CMB is from the cloud of hot plasma that filled the entire universe until a few hundred thousand years after the big bang.

However, the CMB that we on Earth can see comes from a particular part of that cloud – specifically, a thin spherical shell with Earth at the center. That’s the part of the primordial gas cloud that just happened to be the right distance away for its light to be arriving at Earth right now.

As time goes by, the spherical shell emitting the CMB that is arriving at Earth gets larger and larger. It recedes from us faster and faster. Eventually it will redshift out of existence.

One might think a detail such as this would be covered by every cite of more
than about 100 words, but it is not.

However, an hour or so of googling indicates Hamster King and Eburacum45
are right and I was wrong.

[cmyk]

We have to keep in mind, that at one point, every point in the universe was on top of each other.

After about 300,00 years from the Big Bang, the entirety of the universe was a dense, hot ball, of glowing plasma. It wasn’t until this epoch, that the universe cooled down enough, and becoming less dense due to expansion, that these very energetic photons could finally break free from its baryonic counterparts and fly free. The CMB appears to us, only from our vantage in the universe, as being on the edge of the observable universe; it’s photons very dim and diluted, and its wavelengths so redshifted, it’s gone beyond the infrared into the microwave.

If an alien, living on a planet where we perceive the CMB to be, we’re to gaze out into our region of space, they couldn’t see earth or the Milky Way, because they’re looking out 13.7 billion years into the past as well. In fact, they’d see the CMB as it appears from their vantage. To them, our galaxy hasn’t even formed yet, because our light/spacetime reference frame is in their past.

It’s all relative.

[Esox_Lucius]

@eburacum45

Nope; that’s not right. The CMBR comes from the edge of the visible universe.
See this thread for more details.
Cosmic microwave background radiation - Factual Questions - Straight Dope Message Board

I just read your post in that thread and the first sentence is “Note; it is true that the CMBR comes from everywhere, including here; but it does not come from everywhen, but rather it was emitted at a specific time, 13.7 billion years ago.”

The part I bolded is what I was saying, isn’t it? At the time it happened, the universe was filled with the flash of radiation, and has remained filled with it ever since. Hence, it is everywhere. Even our TV sets can pick it up, apparently.

[eburacum45]

Yep; the radiation is everywhere, and it was emitted from everywhere; but the radiation we actually see is 13.7 billion years old. The radiation which was emitted from our location in space is now 46 billion light years away, so we can’t see it.