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 ...
- 113k
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 ...
- 113k
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 ...
- 113k
33
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 ...
- 502
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 (...
- 12.8k
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, ...
- 113k
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 ($\...
- 141k
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, ...
- 6,486
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 ...
- 31.7k
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 ...
- 12.7k
13
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 ...
- 269
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 ...
- 8,038
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 ...
- 36.5k
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} \...
- 141k
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....
- 17.3k
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. ...
- 113k
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 ...
- 141k
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 ...
- 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 ...
- 5,290
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 ...
- 113k
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 ...
- 36.5k
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 ...
- 3,415
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 ...
- 36.5k
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 ...
- 3,922
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 ...
- 3,151
8
votes
Accepted
Does the expansion of space stretch the space-time fabric?
The only physical property of a point in spacetime is its curvature (specified by a many-component tensor). There is no property that tells us "how much it has expanded". So "expanded&...
- 4,261
8
votes
Could we (Earth, Humanity, Solar System) be falling into a black hole?
If a system is falling into (or orbiting) some large mass, like a black hole, it experiences tidal forces. Particles closer to the gravitating mass are pulled toward it more strongly, while particles ...
- 4,261
8
votes
Accepted
Why can't dark energy be considered a 5th fundamental force?
Dark Energy is a scalar field, whereas forces are vector interactions
As far as we know, we are sure that dark energy, or more accurately, the expansion of space-time, is a scalar quantity. That is, ...
- 2,032
7
votes
Does time slow down because the universe is expanding at an accelerating rate?
Yes, time does run slower for far-away objects, as observed from our point of view; this is a prediction of general relativity. And yes, because expansion accelerates, this time dilation slowly, very ...
- 36.5k
7
votes
Does matter accumulate just outside the event horizon of a black hole?
Thought provoking cosmologists!
I'm uber-late to this discussion as I see it has been ongoing for literally years and don't know if there is still anyone monitoring this thread, but here goes.
I ...
- 79
Only top scored, non community-wiki answers of a minimum length are eligible