Questions tagged [space-time]

Questions regarding the 4-dimensional background structure of our universe.

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49 votes
8 answers
10k views

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

My understanding is that time slows and approaches stopping when approaching the event horizon of a black hole. I have seen this explained several places, including a brief explanation in the last ...
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40 votes
3 answers
11k views

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

How do scientists know that distant parts of the universe obey the physical laws exactly as we observe around us? The question might look a bit odd but I am really stuck on my head. We know, ...
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29 votes
2 answers
5k views

Age of the universe and time dilation

Given our knowledge and the standard cosmological model, we estimate that the age of the universe is about 13.7 billion years old. How much sense does it make to talk about the age of the universe as ...
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28 votes
5 answers
9k views

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

The age of the Universe is estimated at 13.8 billion years, and current theory states nothing can exceed the speed of light, which can lead to the incorrect conclusion that the universe can't have a ...
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24 votes
4 answers
4k views

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

So because I can only really think of space-time in 2-dimensions like a sheet of something, my assumptions might be wrong to begin with. I was watching a YouTube video on black-holes and there was a ...
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20 votes
5 answers
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By putting a mirror in space, would we be able to see into the past?

I only ask this because of how fast light travels. The question remains in the title. Why, or why not, would this work?
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19 votes
5 answers
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Is it suspicious that gravitational waves propagate at the speed of light?

Thinking about gravitational waves and the fact that they propagate at the speed of light, I was wondering if it isn't suspicious - the speed of light I mean. Does it perhaps point to something ...
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17 votes
6 answers
6k views

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

If the sun were to suddenly disappear, the planets would continue to travel tangentially to their former orbits. (This I know from this answer to a somewhat related question here.) In such a scenario, ...
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17 votes
4 answers
20k views

What is the difference between our time and space time?

I am trying to understand the phenomenon of space-time. But, everything on internet seems to be too complicated for me since I do not have a background in physics. Can anyone give me simple ...
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16 votes
1 answer
2k views

Time according to the gravity of Sagittarius A*?

This may be a really dumb question (I'm more of a Biologist than an Astronomer) so I apologize in advance for my little knowledge pertaining to Astronomy, but, if I'm not mistaken, time is effected by ...
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14 votes
3 answers
12k views

Would time go by infinitely fast when crossing the event horizon of a black hole?

If you were to fall into a black hole, my understanding is that from your reference point, time would speed up (looking out to the rest of the universe), approaching infinity when approaching the ...
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12 votes
3 answers
15k views

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

So, I watched Interstellar and if you watched it too you know that there's a planet orbiting a black-hole, they call it Miller's Planet. According to the movie, every hour on Miller's Planet is ...
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10 votes
3 answers
329 views

Is there anything special about our position in the galaxy thats necessary for life?

It's a fact that our planet's location from our parent star (among other factors) is of crucial importance to the existence of life. But can our position in the galaxy be considered a factor when ...
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9 votes
3 answers
2k views

How is time defined in astronomy?

I was amazed when my teacher told me that in space, time is define as a coordinate. It means we can define something with a four coordinate system with "negative time" $-t$ as easily as we can say $-...
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9 votes
2 answers
611 views

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

I understand basically that the universe is homogenous (looks the same from every point) and I was told in my astro class that it's supposed to have no center, but how that works boggles my mind. I ...
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9 votes
5 answers
6k views

What is spacetime 'made' of?

General relativity is often explained as saying spacetime is curved by gravity, what does this mean? How could we perceive a curve in spacetime when there is no external "straight" reference frame for ...
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8 votes
1 answer
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What are the stages in the life of a universe?

Geological periods describe varying phases in the last few hundred million years on earth. Are there any names for the periods on a timescale 1-2 orders of magnitude greater, that is, for the stages ...
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8 votes
1 answer
293 views

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

This study found that the universe has a positive curvature https://www.nature.com/articles/s41550-019-0906-9. I didn't really want to buy it and see if it says how big the universe might be. I know ...
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8 votes
1 answer
261 views

What it would look like to observe people with a different time flows?

As I learned, that the bigger gravity source you are influenced by the more slow time ticks for you, the farther away you are from a gravity source the faster times ticks. So Imagine two different ...
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8 votes
1 answer
578 views

Spinning black hole vs non spinning black hole

Does a spinning black hole have a stronger gravity than a non spinning one.After all, a spinning black hole is stretching space-time more, so does that make a difference is gravity?
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8 votes
1 answer
621 views

Universe is expanding, why doesn't time expand with it?

Reading this response When the universe expands does it create new space, matter, or something else? I understand Universe is not expanding like if it is stretching its space, but it is actually ...
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7 votes
4 answers
32k views

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

The Sun's rays hit our eyes around 8 minutes after they are emitted from the Sun. Does this mean that the Sun that we see is always the Sun as it was some 8 minutes before? I strongly think this must ...
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7 votes
3 answers
2k views

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

If we look back far enough we can see all the origins of the universe, so is it possible, even if not feasible, that we could trace the history of some matter as it moves through space-time? I want to ...
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  • 181
7 votes
2 answers
148 views

What Types of Radiation Emanate in the Future and are Perceived in the Present?

Years ago I came upon a scientific text which mentioned different types of radiation and how they are perceived. One such form of radiation was described (from our perspective and understanding of ...
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7 votes
2 answers
453 views

Why do we have the cosmological constant?

Since the cosmological constant is not required to explain that the universe seem to be expanding, why do we have it? What other factors cause us to have that constant? Background: Without the ...
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7 votes
1 answer
1k views

When we say a galaxy is 200 million light years away, does this account for the expansion of space in the time it took it's light to reach us?

When we see a distant galaxy, the light we are watching started it's journey millions of years ago. In all that time, space has been expanding, so if the initial distance was A, right now the distance ...
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7 votes
1 answer
140 views

Does the accelerating expansion of spacetime mean that the pace of time is changing?

Space expands everywhere, also here. And time is inseparable from space. Does this mean that time also "expands" as in changing its pace? Is the changing rate of time also astronomically observable? ...
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7 votes
4 answers
609 views

Distinction between metric expansion and objects just moving apart from each other?

I see explanations likes this: Galaxies and other sources, then, are not strictly expanding away from each other but rather are attached to the fixed grid on the expanding fabric of spacetime. Thus, ...
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6 votes
3 answers
1k views

What shape are black holes theorized to be?

I have always been curious about what black holes (or their event horizons) would look like if we could directly observe them. I notice that they tend to be depicted as flat holes in space but it ...
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6 votes
3 answers
4k views

What is the relative time difference between us and a star system in outer layer of our galaxy?

Just curious to know what will be the time difference between someone living on the outer layer of our galaxy and us, considering the known facts that Time is relative to gravity Our sun goes around ...
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6 votes
4 answers
752 views

Can it be inferred that our cosmological horizon has increased over time?

If I am right, we can see only those stars that lie within our cosmological horizon, and there may or may not be any stars beyond that. Given last 150 years of using telescopes, and since then our ...
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6 votes
2 answers
4k views

Our universe the surface of a 4-dimensional sphere?

The cosmic microwave background that we observe uniformly around us is usually explained by assuming that our universe is the surface of a four dimensional sphere. That way the uniformity makes sense ...
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6 votes
1 answer
185 views

Time dilation on an object circling earth

How would a receiver on Earth hear a radio transmission from an object circling earth at 99% of the speed of light for 24 hours. The transmission from the object circulating would be non-stop. ...
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  • 163
6 votes
2 answers
276 views

Is there an objective difference between space expansion and reduction in speed of light

I just read an article about the cosmological model of an inflationary universe. As you probably know it basically states that space itself is subject to an expansion over time. Since there is no ...
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5 votes
2 answers
896 views

Confused about rubber sheet analogy!

How to resolve confusions on the rubber sheet analogy of the spacetime curvatures? I am a newbie to spacetime curvature. I have watched several youtube videos on Einsteins GR and spacetime curvature ...
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5 votes
2 answers
609 views

Why Do Planets Revolve Faster When they Are Closer to Their Parent Star?

According to Kepler's second law: A planet sweeps out equal areas in equal times (A simple definition). This means that if the orbit is somewhat eccentric, the planet will move faster when on the ...
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5 votes
3 answers
8k views

Does time slow down because the universe is expanding at an accelerating rate?

If the universe is expanding at an accelerating rate, such that the galaxies' moving away from each other is accelerated, then time should also slow down. And when universe will accelerate to the ...
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5 votes
2 answers
2k views

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

We all know Sun gravity, makes the space time curvature and space pushes the earth towards the Sun. But my question is "why the distance between the Sun and Earth is Same? Earth should have moved ...
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  • 63
5 votes
2 answers
283 views

How would Special relativity explain past-present-future if thinking about 13-billion-light-years distanced objects

Halo. I'm interested in how would Special relativity comment on this: Suppose there are 2 objects, A and B. They are 13 billion light years away from each other. When the light from the B object ...
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5 votes
2 answers
2k views

What effect does time dilation have on bodies orbiting close to Black Holes?

As far as my (somewhat basic) knowledge of astrophysics goes in general the closer to a star your orbit gets smaller (because you travel less distance) and faster (because you're deeper in the gravity ...
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5 votes
2 answers
279 views

Questions about time and space (from beginner)

To be honest my knowledge about astronomy and related physics is close to zero. However, since I studied math a bit, I would like to ask a couple maybe stupid questions, which I am curious about and I ...
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  • 153
5 votes
1 answer
2k views

What is the actual black hole merger speed?

I have seen models of 2 black hole mergers. https://www.theverge.com/2017/11/16/16665132/gravitational-waves-ligo-virgo-black-holes-merger-multi-messenger-astronomy When we see a space ship appear ...
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  • 1
5 votes
2 answers
137 views

What is the difference between the way light travels in a gravitational field and the way it travels through an expanding universe?

A photon is travelling through space. As it travels, the space through which it travels is expanding Does this describe A - a photon travelling from a distant galaxy to earth B - a photon travelling ...
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  • 191
5 votes
1 answer
235 views

Can Space-Time Itself Have Energy Qualities Like Momentum?

Hypothetically, if space could be moved or scaled, what would happen to everything inside? I'd like to know if an element of momentum could be transferred to objects by contorting space-time. Other ...
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  • 231
5 votes
3 answers
300 views

Theoretically could the expansion of the universe affect acceleration due to gravity?

I was wondering if theoretically the universe expanding affects speed or acceleration due to gravity, compared to if space weren't accelerating. The way I have it in my head, something is falling ...
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  • 51
5 votes
0 answers
251 views

Gravitational eddies across the galaxy?

I read about natural gravitational eddies that travel in a wave that black holes have. They also have a strong magnetic field. Does those eddies follow magnetic field lines of the rotating black hole? ...
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4 votes
2 answers
4k views

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

How long would it take to reach the current edge of the reachable portion of the universe, with the following bounds in mind: We figure out a way build a space ship to provide a constant 1 G ...
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4 votes
1 answer
2k views

Can a black hole rip spacetime

Can a black hole which has an enormous mass and spins so fast cause spacetime to to be stretched to is absolute limits that it start so rip?
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4 votes
1 answer
197 views

Is it possible to overcome the problem of blind spot(s) of current gravitational wave detectors?

If gravitational waves (GWs) pass through specific points (which are known as blind spots), current GW detectors aren't able to detect the passing waves. In the future, will we be able to completely ...
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  • 341
4 votes
2 answers
140 views

Neutron star size in different reference frames

What is the estimated size of neutron stars observed in their reference frame and in our reference frame? That is, how bent is space-time around neutron stars?
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