Starship Asterisk*

Chris Peterson wrote: ↑Wed Mar 16, 2022 9:26 pm

johnnydeep wrote: ↑Wed Mar 16, 2022 9:06 pm What does this statement mean:

Some neutrinos and gravitational waves that surround us come from even farther out, but humanity does not yet have the technology to detect them.

We commonly treat the edge of the observable universe as the horizon where it was opaque to electromagnetic radiation. But the actual edge is a bit beyond that, where it is receding from us at greater than c. Things like gravitational waves from beyond the opaque edge can reach us, and could theoretically give us the ability to collect information right to the true observability horizon.

JohnD wrote: ↑Thu Mar 17, 2022 1:46 pm Eh? You are " discussing nothing here but our universe. No others."

But Chris, you said/wrote, " And indeed, there are observable universes with no contents at all in common." Where are they, how can you observe them? You cannot be referring to the Universes observed by you and by anyone else, as their contents will be the same.

But I would agree with your prior sentence, that "the closer together we are, the more of those contents that will be the same" so perhaps I fail to see what you mean by the second.
John

JohnD wrote: ↑Thu Mar 17, 2022 9:01 am

Chris Peterson wrote: ↑Wed Mar 16, 2022 7:10 pm We all have different observable universes, with different contents. The closer together we are, the more of those contents that will be the same (probability is not involved). But never all of them. And indeed, there are observable universes with no contents at all in common.

Wow! Really? You can see into universes other than our own?

Ann wrote: ↑Thu Mar 17, 2022 5:27 am

Chris Peterson wrote: ↑Wed Mar 16, 2022 7:50 pm

lefthip wrote: ↑Wed Mar 16, 2022 7:44 pm As a believer in the scientific principle, the universe can only be that which we can detect by sight or otherwise. Multiverses or the infinite universe are only conjectures, although reasonable hypotheses, and may ultimately prove to be correct. But, right now our universe is a sphere of about 13.5 billion light years diameter.
Or is it? Do we know where we sit in this sphere in relation to its centre? I would assume that it is not the centre, as that would be very statistically unlikely. So, if we can "see" 13.5 billion light years, shouldn't we be able to say with some significant degree of certainty that the universe is actually much larger. We have discovered the redshift which suggests the all galaxies are moving away from each other. Couldn't these redshifts be triangulated to point to a centre and calculate its position (in relation to everything else)?
+

Theory and a wealth of supportive evidence argue against the Universe being a sphere. An observable universe is spherical. Not the Universe. In three dimensions, the Universe has no center and no edges, although we can reasonably argue that every 3D point within it represents a center point with respect to everything else. The only "edge" that makes any sense is the 3D surface that everything exists on, and which is expanding outward along the time axis... an axis we have no access to in either direction.

Theory can reasonably support either an open or closed universe, a finite or an infinite universe. The jury is still out in that regard. But we can very reasonably say that the edge of the observable universe lies 46 billion light years away, not 13.5 billion.

Chris, haven't you said that the edge of the Universe is the Big Bang (or whatever we choose to call the very beginning of the Universe - some say that inflation came before the Big Bang)?

As for whether the Universe is open or closed, shouldn't we, at least as a starting point, use Occam's Razor and choose the simplest answer? So that, if the Universe is flat, it is most likely open (and only topology can change that)?

Of course, we are not sure if the Universe is really, truly, absolutely flat. It could be marginally, marginally spherical, and therefore finite and closed.

Chris Peterson wrote: ↑Wed Mar 16, 2022 7:10 pm We all have different observable universes, with different contents. The closer together we are, the more of those contents that will be the same (probability is not involved). But never all of them. And indeed, there are observable universes with no contents at all in common.

Chris Peterson wrote: ↑Wed Mar 16, 2022 7:50 pm

lefthip wrote: ↑Wed Mar 16, 2022 7:44 pm As a believer in the scientific principle, the universe can only be that which we can detect by sight or otherwise. Multiverses or the infinite universe are only conjectures, although reasonable hypotheses, and may ultimately prove to be correct. But, right now our universe is a sphere of about 13.5 billion light years diameter.
Or is it? Do we know where we sit in this sphere in relation to its centre? I would assume that it is not the centre, as that would be very statistically unlikely. So, if we can "see" 13.5 billion light years, shouldn't we be able to say with some significant degree of certainty that the universe is actually much larger. We have discovered the redshift which suggests the all galaxies are moving away from each other. Couldn't these redshifts be triangulated to point to a centre and calculate its position (in relation to everything else)?
+

Theory and a wealth of supportive evidence argue against the Universe being a sphere. An observable universe is spherical. Not the Universe. In three dimensions, the Universe has no center and no edges, although we can reasonably argue that every 3D point within it represents a center point with respect to everything else. The only "edge" that makes any sense is the 3D surface that everything exists on, and which is expanding outward along the time axis... an axis we have no access to in either direction.

Theory can reasonably support either an open or closed universe, a finite or an infinite universe. The jury is still out in that regard. But we can very reasonably say that the edge of the observable universe lies 46 billion light years away, not 13.5 billion.

johnnydeep wrote: ↑Wed Mar 16, 2022 9:06 pm What does this statement mean:

Some neutrinos and gravitational waves that surround us come from even farther out, but humanity does not yet have the technology to detect them.

Some neutrinos and gravitational waves that surround us come from even farther out, but humanity does not yet have the technology to detect them.

Ann wrote: ↑Wed Mar 16, 2022 6:05 pm

Guest wrote: ↑Wed Mar 16, 2022 1:41 pm Why does it seem that M66 is listed twice?

Good point. Because the artist behind the image likes it so much? M66

(Or he got distracted - darn, I forgot to buy milk, and I bet I didn't include M66 among the galaxies in my picture of the observable Universe - better fix that right away...)

Ann

lefthip wrote: ↑Wed Mar 16, 2022 7:44 pm As a believer in the scientific principle, the universe can only be that which we can detect by sight or otherwise. Multiverses or the infinite universe are only conjectures, although reasonable hypotheses, and may ultimately prove to be correct. But, right now our universe is a sphere of about 13.5 billion light years diameter.
Or is it? Do we know where we sit in this sphere in relation to its centre? I would assume that it is not the centre, as that would be very statistically unlikely. So, if we can "see" 13.5 billion light years, shouldn't we be able to say with some significant degree of certainty that the universe is actually much larger. We have discovered the redshift which suggests the all galaxies are moving away from each other. Couldn't these redshifts be triangulated to point to a centre and calculate its position (in relation to everything else)?
+

JohnD wrote: ↑Wed Mar 16, 2022 6:43 pm

Chris Peterson wrote: ↑Wed Mar 16, 2022 2:39 pm What? That we are each at the center of a unique and different observable universe is a simple fact. There is no alternative explanation. An observable universe is defined by a sphere of a certain radius around a 3D point. There are an infinite number of such center points in the Universe. The apparent edge of the universe I see is different from the apparent edge of the one you see.

(Whether we are all at our own centers of the Universe as a whole depends upon how we define "center" and on the topology of the Universe.)

Ah! If you would care to edit your statement to read that every one has a different edge to their universe, then I will agree, but the contents will be identical, or as near identical as makes no difference. The probability that there will be a significant difference between our universes is infinitesimally small. In fact a great deal smaller than you could split a hair!
John

Chris Peterson wrote: ↑Wed Mar 16, 2022 2:39 pm What? That we are each at the center of a unique and different observable universe is a simple fact. There is no alternative explanation. An observable universe is defined by a sphere of a certain radius around a 3D point. There are an infinite number of such center points in the Universe. The apparent edge of the universe I see is different from the apparent edge of the one you see.

(Whether we are all at our own centers of the Universe as a whole depends upon how we define "center" and on the topology of the Universe.)

Guest wrote: ↑Wed Mar 16, 2022 1:41 pm Why does it seem that M66 is listed twice?

Guest wrote: ↑Wed Mar 16, 2022 4:39 pm I think it is funny and very narcissistic that we are always placed in the center of the universe. How do we know we are not at an "edge"?

MarkBour wrote: ↑Wed Mar 16, 2022 3:48 pm

NCTom wrote: ↑Wed Mar 16, 2022 2:56 pm This is a fantastic APOD. Thank you also for all the comments. I have often wondered about the phraseology used to define the limits of our universe. This makes it more reasonable that our "observable" universe is 13.4 billion years old rather than "the" universe is 13.4 billion years old. This should indicate as our technical skills increase, the age and size of our "observable" universe may also increase leading to the necessity of rewriting our science books!

I wholeheartedly agree! And I'm placing my bet that the James Webb Telescope (JWST) is going to observe some things that puncture our current view of how things are. I don't know if it's possible, but if it images a highly red-shifted galaxy that figures to be 20 billion years old, that'll be a great moment.

NCTom wrote: ↑Wed Mar 16, 2022 2:56 pm This is a fantastic APOD. Thank you also for all the comments. I have often wondered about the phraseology used to define the limits of our universe. This makes it more reasonable that our "observable" universe is 13.4 billion years old rather than "the" universe is 13.4 billion years old. This should indicate as our technical skills increase, the age and size of our "observable" universe may also increase leading to the necessity of rewriting our science books!

Chris Peterson wrote: ↑Wed Mar 16, 2022 4:37 am

APOD Robot wrote: ↑Wed Mar 16, 2022 4:06 am Explanation: How far can you see? Everything you can see, and everything you could possibly see, right now, assuming your eyes could detect all types of radiations around you -- is the observable universe.

Not the observable universe, but an observable universe. Every observer in the Universe has their own observable universe, all different.

NCTom wrote: ↑Wed Mar 16, 2022 2:56 pm This is a fantastic APOD. Thank you also for all the comments. I have often wondered about the phraseology used to define the limits of our universe. This makes it more reasonable that our "observable" universe is 13.4 billion years old rather than "the" universe is 13.4 billion years old. This should indicate as our technical skills increase, the age and size of our "observable" universe may also increase leading to the necessity of rewriting our science books!