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 ...
  • 93.4k
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 ...
  • 93.4k
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 (...
29 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 surface ...
  • 462
25 votes
Accepted

Time according to the gravity of Sagittarius A*?

Not at all a dumb question. As you have heard, it is true that time is affected by gravity. The stronger the gravitational field, the slower time passes. If you're far from any gravitating matter, ...
  • 34.1k
22 votes
Accepted

When we see the Sun, do we actually see its past?

Yes, you are right. We don't only see the Sun 8 minutes in the past, we actually see the past of everything in space. We even see our closest companion, the Moon, 1 second in the past. The further an ...
  • 1,901
19 votes
Accepted

Age of the universe and time dilation

The answer is yes time dilation does affect how much time an observer experiences since the big bang until the present (cosmological) time. However there is a certain set of special observers called ...
  • 1,805
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 ($\...
  • 120k
16 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, ...
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 ...
  • 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 ...
  • 10.5k
13 votes
Accepted

Life planets orbiting black-holes. Can/Do they really exist?

Well, first things first. It's not likely to have a planet orbiting near a black hole and in significant time dilation because the tidal effects would likely tear anything that close apart. ...
  • 22.9k
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 ...
12 votes

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

We need to think about just where the time dilation effect occurs. By then thinking about the observations from each point of view, that is the free falling object and the external observer, we can ...
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 ...
  • 34.1k
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} \...
  • 120k
10 votes

What is a singularity? What is at the center of a black hole? Specifically regarding space-time

As others have said, mathematically, a singularity is when there is an attempt to divide by zero. Take, for example a Schwarzschild black hole. This is a black hole that has no electric charge or ...
  • 34.2k
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. ...
  • 93.4k
9 votes

Age of the universe and time dilation

In the standard model, the universe looks the same for all locations moving in the local rest frame. This includes its apparent age. You can tell if you are in the local rest frame if the expansion ...
  • 2,857
9 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 ‘...
  • 91
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 ...
  • 93.4k
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 ...
  • 34.1k
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 ...
  • 3,226
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 ...
  • 34.1k
8 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....
  • 16.8k
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 ...
  • 3,291
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 ...
  • 3,102
7 votes
Accepted

Why does the distance between Sun and Earth stay the same?

While the Sun and Earth attract each other, they cannot fall into each other because of angular momentum conservation. In a central field (where the force is acts in the direction of the distance ...
  • 5,306
7 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 ...
  • 5,180
7 votes

Life planets orbiting black-holes. Can/Do they really exist?

Interstellar's "Miller's Planet" is utter rubbish... First of all, black holes don't start out as black holes. Black holes form at the end of the life cycle of very big star (at least 25 solar masses ...

Only top scored, non community-wiki answers of a minimum length are eligible