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
  • 122k
42 votes
Accepted

Is it suspicious that gravitational waves propagate at the speed of light?

It is very suspicious! It points to the fact that the speed of light isn't just some random speed that light happens to travel at, but is a fundamental property of the universe. In fact, any massless ...
James K's user avatar
  • 122k
42 votes
Accepted

Why is time referred to as "The" capital T 4th dimension?

This is because time is the fourth dimension in the theory of General Relativity which describes gravity. It turns out that a good way to describe the paths that objects or light take when in a ...
James K's user avatar
  • 122k
32 votes
Accepted

Does matter accumulate just outside the event horizon of a black hole?

Yes, you are absolutely right, from OUR VIEWPOINT it does. From Kip Thorne's book "Black Holes and Time Warps: Einstein's Outrageous Legacy." Like a rock dropped from a rooftop, the star’s ...
ctrebor's user avatar
  • 498
32 votes
Accepted

If the Sun disappeared, could some planets form a new orbital system?

The issue here is whether pairs of planets can become gravitationally bound to each other. In the two-body problem the trajectories or orbits are ellipses (bound orbits), parabolas and hyperbolas (...
Anders Sandberg's user avatar
22 votes

Does seeing a gravitationally lensed/magnified galaxy imply that they could also see us as well?

It is highly unlikely that a galaxy that we see as magnified by lensing is, has, or will be positioned to see our galaxy similarly magnified. The three galaxies, including the lensing galaxy or ...
eshaya's user avatar
  • 3,956
18 votes

Could we (Earth, Humanity, Solar System) be falling into a black hole?

No. There are two ways of understanding your question. One is "could there be a nearby, relatively small black hole (say, a few million times the mass of the sun) into which we are falling. No, ...
James K's user avatar
  • 122k
17 votes

If an exoplanet transit we are seeing is 13000 light years away, are we seeing a 13000-year-old orbit?

In one word, yes. Anything and everything we see, we see the way it was a certain time ago—about 1.3 seconds for the Moon, about 13,000 years for your hypothetical planet. Like @Richyt pointed out, ...
Pierre Paquette's user avatar
17 votes

What are the stages in the life of a universe?

Yes there are. They are mainly based on what dominates the energy density of the universe at the time and they are known as epochs. Thus we have the inflationary epoch in the first tiny fraction ($\...
ProfRob's user avatar
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17 votes

How do we work out the light travel time on a cosmic scale?

Let's first be clear that there is no unique way to identify the time or distance between two events. This is true in every relativistic context; just think about relativistic time dilation and length ...
Sten's user avatar
  • 4,664
15 votes

How is time defined in astronomy?

$t$ signifies time; see the Wikipedia article for spacetime, and then the subsection for 4-vectors. The basics are pretty natural to understand. Suppose something happens, an event, like an apple ...
uhoh's user avatar
  • 30.7k
14 votes
Accepted

Can we track matter through time by looking at different depths in space?

Would it be possible to look deep into a certain part of space and time to find some galaxy that contributed to the matter that makes up the Milky Way today? No, that's not possible. If we could ...
PM 2Ring's user avatar
  • 14.5k
13 votes
Accepted

How does the concept of a universe with no center work?

When we talk about the universe, we are really talking about one of two things: The observable universe, which is everything we can possibly see. The Universe, which is everything that has ever ...
Sir Cumference's user avatar
12 votes
Accepted

What are the dimensions of LIGO Detector evacuated chambers?

The arms are $4\,\mathrm{km}\,\times\, 1.2\,\mathrm{m}$: From the LIGO webpage: The 1.2 m diameter beam tubes were created in 19-20 m-long segments, rolled into a tube with a continuous spiral ...
pela's user avatar
  • 38.3k
12 votes

Is it suspicious that gravitational waves propagate at the speed of light?

The perturbation to the metric of spacetime (known as the strain), caused (for example) by an oscillating mass quadrupole, obeys a wave equation of the form $$ \nabla^2 h^{\mu \nu} = \frac{1}{c^2} \...
ProfRob's user avatar
  • 152k
12 votes

Is the gravitational constant really constant over our Universe?

See also Do the laws of physics work everywhere in the universe? on the Physics SE. If the gravitational constant were not constant, then the laws of physics work differently elsewhere in the universe....
Allure's user avatar
  • 4,544
11 votes
Accepted

Why we can't see our Sun as it was a few years ago?

Our Sun moves through the galaxy much slower than light does. So the light that was emitted last year by the Sun is already very far away from the Earth. Therefore, we don't receive any light from (i....
usernumber's user avatar
  • 17.5k
10 votes
Accepted

Confused about rubber sheet analogy!

The rubber sheet only is not meant to be a qualitative model, it gives one concept and one concept only: Mass causes curvature of spacetime. You can't get any more than that from the rubber sheet. ...
James K's user avatar
  • 122k
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
  • 152k
10 votes

Why is time referred to as "The" capital T 4th dimension?

For pretty much the same reason that the first 3 are (x, y, z) and not (Hair Colour, Body Type, Salary). It's part physics and part linguistics. As @JamesK said x,y,z,t are the most useful to describe ...
Aubreal's user avatar
  • 201
9 votes

Does matter accumulate just outside the event horizon of a black hole?

Several wonderful yet technical answers have been given, and I cannot add anything to those very nice answers that explain why it is not useful to think black holes get "frozen" at their event ...
Ken G's user avatar
  • 5,330
9 votes

Does seeing a gravitationally lensed/magnified galaxy imply that they could also see us as well?

It depends on the distance between us, the lens galaxy, the magnified galaxy, and many other factors due to the way light travels. A lens galaxy passes the old light of a far-off galaxy and amplifies ...
4NT4R3S's user avatar
  • 899
8 votes
Accepted

Can a black hole rip spacetime

There is a useful model of spacetime as a rubber sheet that is bent by masses laying on it. But it should be remembered that this is an analogy (Obligatory xkcd) and most analogies fail if pushed too ...
James K's user avatar
  • 122k
8 votes

Why is the observable Universe larger than its age would suggest?

I was just thinking about that and here is my layman's explanation. Imagine you're tracing two dots on a crumpled piece of paper, the dots are moving, but as they are moving, so is the paper getting ‘...
Hany's user avatar
  • 97
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
  • 38.3k
8 votes

What is the oldest thing?

Pretty much every hydrogen atom that's in a glass of water has a proton that dates from 1 / 1000000 seconds after the big bang. That's older than the cosmic microwave background, which dates from ...
antlersoft's user avatar
  • 3,455
8 votes

How does time work beyond the cosmic event horizon?

First, let's clear up a few misconceptions: The Hubble sphere The speed of light as an upper limit is valid in special relativity (SR). In general relativity (GR), which must be used to describe the ...
pela's user avatar
  • 38.3k
8 votes
Accepted

Based off of the study that found the universe has a positive curvature, how big would the whole universe be?

Surprisingly small! (To me, at least.) The paper referred can be found on the arXiv as Di Valentino et al. (2019). As is customary with Planck results, the exact values of the cosmological ...
pela's user avatar
  • 38.3k
8 votes

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

See also: Do the laws of physics work everywhere in the universe? Noether's theorem, in the context of this question, states that: If the laws of physics do not vary with position, then linear ...
Allure's user avatar
  • 4,544
8 votes

If the Sun disappeared, could some planets form a new orbital system?

No. The 8 planets would go into 8 different directions. It is because their relative velocity to each other is much higher than the escape velocity, even from their smallest distance. If it would not ...
peterh's user avatar
  • 3,161

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