54 votes
Accepted

How do scientists know that the distant parts of the universe obey the physical laws exactly as we observe around us?

We don't know in general but to the extent we can measure, the laws seem to be the same, even if conditions are not. For example radioactive decay: We know how fast various elements decay, and we can ...
James K's user avatar
  • 118k
50 votes
Accepted

What is the hottest thing in the universe?

Energetic neutrinos have been observed from the core of a supernova (SN 1987A). The inferred temperature at the "neutrinosphere" is about 4 MeV (equivalent to 50 billion K - ($5\times 10^{10}...
ProfRob's user avatar
  • 149k
49 votes
Accepted

How can telescopes see anything at all?

Yes, space is very empty. There is not nothing between us an the Eagle nebula, but little enough that we can still get a reasonable view of it. The pillars are ephemeral, they are evolving on ...
ProfRob's user avatar
  • 149k
36 votes
Accepted

How much larger will the "observable by us" universe be when JWST becomes operational?

tl;dr Conservatively 10%, realistically 25%, optimistically 60%. I assume that by the "observable-by-us Universe", you mean not the theoretically observable Universe, which is given by the ...
pela's user avatar
  • 37.6k
27 votes
Accepted

Does the recent news of "ten times more galaxies" imply that there is correspondingly less dark matter?

All Conselice et al. (2016) appear to suggest is that when you look at something like the Hubble deep field, there are many faint (and presumably low mass) galaxies that are not seen. This has ...
ProfRob's user avatar
  • 149k
26 votes

What is the hottest thing in the universe?

Note that while we haven't observed anything even close, there is a theorized Absolute Hot along the lines of absolute zero. Its theorized value is ~ $1.416 \cdot 10^{32}$ Kelvin. Above this ...
SoronelHaetir's user avatar
26 votes

Is there anything currently 46 billion light years away from Earth that we can see?

The CMBR came from a sphere of matter with an extrapolated comoving radius of around 46 billion ly. That's the most distant thing we can see. The observable universe is sometimes defined to end there, ...
benrg's user avatar
  • 3,822
25 votes
Accepted

Why can we observe the Cosmic Microwave Background no matter the direction we look?

Until the Universe was 380,000 years old, it was filled with a gas of protons an electrons. There was also radiation, in thermal equilibrium with the matter, and because it was so hot, the protons and ...
pela's user avatar
  • 37.6k
24 votes
Accepted

Approximately what percent of the sky has nothing in it?

It is really quite hard to answer the question as posed because as you observe deeper and deeper (e.g. using a larger telescope or observing for longer) then more and more (fainter) objects become ...
ProfRob's user avatar
  • 149k
23 votes

Are there any galaxies which fell out of sight horizon due to cosmic expansion?

No. In fact the opposite is the case. (See the last paragraph for an intuitive explanation.) It is a common misbelief that galaxies receding faster than the speed of light are not visible to us. ...
pela's user avatar
  • 37.6k
22 votes

How can telescopes see anything at all?

Does that mean that between those 7,000 light-years, there's absolute nothing in the way that obscures the pillars? Given it's arcseconds, I'd expect that any kind of disturbance would make it simply ...
gomennathan's user avatar
20 votes

Is there anything currently 46 billion light years away from Earth that we can see?

We don't see stars and galaxies at a proper distance of 46 Gly, because this distance corresponds to a light travel time of 13.7 billion years, or very shortly after the big bang. When we look into ...
James K's user avatar
  • 118k
19 votes
Accepted

As of now how much larger is our practically observable universe compared to just prior to JWST becoming operational

The current redshift record is set by the JADES galaxies. The highest-redshift one at $z = 13.20$ isn't too convincing tbh, but there's one at 12.6 that looks better, and the one at $z = 11.6$ should ...
pela's user avatar
  • 37.6k
18 votes
Accepted

Could the redshift of all incoming photons be explained by a massive ring of distant masses pulling the sources of the photons away?

You have identified the issues. The model does not explain the redshift-distance relationship, which is one of the primary pieces of evidence. Simply to say "our ideas about gravity are wrong&...
ProfRob's user avatar
  • 149k
17 votes

Approximately what percent of the sky has nothing in it?

Zero. At the limit, there's the Cosmic Microwave Background which represents the content of the Universe not long after it came into being. Of course, some would no doubt propose that since the CMB ...
Mark Morgan Lloyd's user avatar
12 votes

Logically, how can the universe be infinite in size?

Something infinite can expand. Consider an infinite length of elastic. There are (infinitely) beads attached to it at 1m gaps. You might label one of the beads "0", then the next one is &...
James K's user avatar
  • 118k
11 votes

Photon Paradox?

And that is why you don't do the calculations in a frame that is moving at lightspeed. If you have two observers that are moving relative to each other you can use the Lorentz transformation to ...
James K's user avatar
  • 118k
11 votes
Accepted

Does the age of the universe take into account General Relativity / Special Relativity?

Rob Jeffries gives a good response to this question, but I wanted to go through the basic outline of how the age of the universe is calculated, just so you can see how it works more or less. Be warned ...
zephyr's user avatar
  • 15k
11 votes

Does the age of the universe take into account General Relativity / Special Relativity?

This is a confusing question - your title mentions GR, but of course the age of the universe is entirely derived as a result of using GR to solve for the dynamics of the expanding universe. The text ...
ProfRob's user avatar
  • 149k
11 votes
Accepted

If we could fly off edge of observable universe what would happen?

The 'edge' of the observable universe is as much a edge as is the 'edge' of how far you can look from the roof of your house: none at all, it's just a limit to our vision. We can never reach this edge ...
planetmaker's user avatar
  • 18.9k
10 votes
Accepted

Is there a physical limit to how far we can go?

Your 3 points are spot on. There is a finite number of galaxies we could theoretically reach for the reasons you say. The further away the distant galaxy the greater the expansion of space between ...
userLTK's user avatar
  • 23.9k
10 votes

Is the age of the Universe really 13.8 billion years?

The age of the universe is not calculated based on the size of the visible universe. The age of the universe is being calculated based on the fact that the laws of nature have no direction. This means ...
MacUserT's user avatar
  • 889
10 votes
Accepted

Would an observer standing at the edge of the "observable universe" perceive the expansion of space-time?

If you move to a position 46 billion light years away, the universe probably looks almost exactly the same as it does here (at least on large scales). The only reason I add "probably", is ...
ProfRob's user avatar
  • 149k
9 votes
Accepted

Where is the North of the Universe

Sort of... There is a system called the International Celestial Reference System (ICRS) which has center at the Solar System Barycenter (normally inside the Sun but not the same as the Sun's center) ...
astrosnapper's user avatar
  • 8,347
9 votes
Accepted

Does the universe curve in on itself?

In the currently dominant theories, the Universe is basically the same everywhere, if you look on a large enough scale. There may not be a furthest star from Earth at all (the universe may be infinite)...
Steve Linton's user avatar
  • 10.3k
9 votes

How is the gravitational effect of galaxies outside of the visible universe on galaxies within the visible universe currently modeled?

We don't know… We don't know how much Universe there is outside our observable Universe. The observable Universe seems to have a "flat" geometry (in the 3D sense, not in 2D). If it really is ...
pela's user avatar
  • 37.6k
9 votes

Rotation of far away and near by galaxies

It depends a bit on what you means by "far away" and "the same", but: Galaxy formation Galaxies form from collapsing and colliding clouds of gas and dark matter in the early ...
pela's user avatar
  • 37.6k
9 votes
Accepted

How is observable matter distributed in the universe?

I think the following is a fair summary. It's based on a fairly old study by Fukugita & Peebles (2004) but the numbers are quite reasonable. Your guess about stars dominating is way off. Most of ...
ProfRob's user avatar
  • 149k
8 votes
Accepted

How long would it take to reach the edge of the reachable universe?

Jonathan's answer is essentially correct, but as Rob Jeffries comments, he doesn't take into account that the Universe is expanding during the journey. The edge of the observable Universe is 47 ...
pela's user avatar
  • 37.6k
8 votes

Are there any galaxies which fell out of sight horizon due to cosmic expansion?

As time passes, there are galaxies that are currently not in the observable universe which will become observable But this is not a sudden winking on. Instead, over hundreds of millions of years we ...
James K's user avatar
  • 118k

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