# Questions tagged [expansion]

Questions about the increase with time of distance between two distant points in the Universe.

236 questions
Filter by
Sorted by
Tagged with
2k views

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

I just read this article in the AUSTRALIAN SKY & TELESCOPE magazine, Nov/Dec 2022 Issue 140, on P16, KEEP YOUR DISTANCE: How far away are the objects we see in the universe? And on P23: "And ...
• 123
151 views

### Why is v lowercase and D uppercase in $v=H_0D$?

Why is v almost always written in lowercase and D in uppercase in $v=H_0D$? OK, v is in lowercase, as usual, but then why is D in uppercase? What's so different/special about it? In my physics school ...
• 123
86 views

### Does a cosmological redshift occur within our local group, of galaxies?

I know that it is not supposed to be expanding but space is space and it is expanding everywhere else outside it so why not inside it too, and gravity just holds everything back. I suppose if it is, ...
• 123
1 vote
1k views

### How can Hubble's original data show that the universe is expanding?

The diagram from Hubble's original data only goes from 0 to 2 Mpc (see here), and yet our Local Group is not supposed to be expanding and it is 1.5 Mpc in radius. How can the data show that the ...
• 123
176 views

### What is Hubble's parameter/constant H0 supposed to mean, exactly? [duplicate]

Surely it must mean that every second each Mpc expands by that much. Well what else, could it be. So if the expansion is accelerating, then how can H0 be decreasing!? Let's look at Hubble's law: v=H0D,...
• 123
127 views

### Why are distant type Ia supernovae dimmer than expected?

Why are the (extremely) distant type Ia supernovae dimmer than expected? Relative to what? Their cosmological red shift? Well, what else. Please don't just say "Because the expansion is ...
• 123
1 vote
179 views

### Is the distance covered always equal to the time taken?

Is the distance covered by light from an extremely distant source in light years equal to the time taken in years? Surely it would have to be, for the speed of light is absolute, after all.
• 123
65 views

### Can we measure how fast space is expanding from how quickly gravitationally lensed galaxies or stars are blurring or getting sharper?

A galaxy that lenses another celestial body should as it moves away from Earth alter the sharpness of what it lenses by gravity. Could this be used to determine how quickly space is expanding?
60 views

### Ejected galaxies, dynamical friction and dark energy?

I have a question after reading this paper (https://arxiv.org/abs/1411.5860). There the authors analyse the ejection of galaxies from their local groups in the presence of dark energy. I had some ...
• 827
1 vote
69 views

### Why is the Hubble time same with age of the universe assuming ideally constant expansion?

Practically, the Hubble time does not perfectly same with age of the universe. But that's considered to coincide when the rate of expansion is constant. However, the constant rate does not mean ...
• 19
1 vote
67 views

### Does dark energy increase eccentricity in orbits of galaxies?

I came across this thesis dissertation which indicates that if a pair of orbiting galaxies are sufficiently far apart (with a sufficiently large orbital radius) then their orbits would change from ...
• 827
152 views

### Why is the Milky Way today about the same diameter as the visible universe was when recombination happened 378000 years after the Big Bang?

Has the Milky Way undergone an expansion of space time independent of the universe as a whole or did the gas that created the Milky Way heat up and expand? Whatever the answer, why the current ...
2k views

### How long has it been since dark energy started to reverse our slowdown and accelerate the expansion of the universe? 4 billion years ago? 4.5? 5?

Or has it been as long as six billion years? Over the last few years I keep seeing longer and longer numbers... Have researchers noticed something different recently? Or found a mistake in their ...
• 4,927
3k views

### Why can't dark energy be considered a 5th fundamental force?

So I have recently been researching for multiple articles about fundamental forces, currently there are just 4 forces: Gravity Electromagnetism Strong nuclear force Weak nuclear force So far there ...
• 2,613
1 vote
71 views

### ER=EPR and information fuelling cosmic expansion

I apologize in advance for the lack of education, I'm a Canadian hillbilly. I was considering the concepts of ER=EPR and the growth of the black hole's internal system with the addition of information ...
50k views

### Shouldn't very very distant objects appear magnified?

My understanding of the expansion of space itself is very shaky, but if space itself is expanding, then shouldn't very very distant objects appear magnified? For example these James Webb Space ...
1 vote
74 views

### Could both the high incidence of supermassive Black Holes (BH) and the expansion of the universe be consequences of vacuum entanglement energy?

The notion that spacetime may emerge from entanglement between factors comprising a Hilbert space decomposition of the vacuum has been suggested by many (for example, M. Van Raamsdonk “Building up ...
• 33
3k views

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

With the expansion of the universe, all objects far away from us appear to be moving away (ie: exhibit Redshift). With distance, this acceleration / redshift also seems to increase (~73,000 (m/s) / (...
• 299
1 vote
85 views

### If astronomers didn't know about redshift could we still determine the universe is expanding?

What other evidence is there for the expansion of the universe? Would the equations of physics look wrong without it?
1 vote
410 views

### Does space expand?

I know this has been asked and here are the links of at least two. The first link below is what I thought was the standard description. What does it mean for space to expand? Now by accident while ...
• 1,161
2k views

### How far apart has the sun drifted from Alpha Centari due to the expansion of the universe since its formation?

From what I read Alpha Centauri formed 250 million years before the sun which was 4.85 billion years. So how far apart were they when the sun formed vs what they are now? The expansion of the universe ...
• 1,161
7k views

### How do we know the expansion of the universe is not centered around our position?

It is my understanding that the red shift of galaxies in whatever direction we look shows the universe is expanding in every direction around us. That could be explained in two ways: A (accepted): The ...
• 393
77 views

### Seeking Guidance & Tips for Collaboration on a Cosmological Project Involving Potential Energy Variation Due to Cosmic Expansion

I'm a student with a deep interest in cosmology and astrophysics. I've embarked on a project and am seeking some guidance, insights, or even ways that I could look for possible collaboration on this ...
1k views

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

ChatGPT and wikipedia have informed me that the primary evidence for the theory that the universe is expanding is the fact that photons that arrive to our planet from all directions are being ...
• 231
1 vote
189 views

### Critical density of universe with intrinsic spatial curvature

$H^2 = \frac{8 \pi G}{3}\rho-\frac{kc^2}{a^2}$ Shown above is the first Friedmann equation. I understand that the curvature parameter, k, represents the spatial curvature of the universe which is ...
• 23
170 views

### where are we in the universe as compared to the observable universe? [duplicate]

For distant galaxies to be accelerating away at equal speed in every direction, Earth would have to be at the centre of the universe. Since it is not, some region of the distant galaxies should appear ...
246 views

### Conditions in the Early Universe

I am interested in how the large-scale properties of the Universe change over time. Does anyone know a trusted website which gives the temperature, density and radius (distance to the particle horizon)...
• 371
147 views

### Can the expansion of the universe cool my beer?

So the expansion of the universe stretches the light traveling through the void, as demonstrated by the cosmic microwave background radiation. These photons are lower energy than when they are ...
4k views

### What equation tells you how far in space you can go from a point and return?

We know that hubble expansion increases the distance between points in space, and that the cosmological event horizon represents the distance from the observer at which objects are receding faster ...
• 313
1 vote
276 views

### Does the expansion of space stretch the space-time fabric?

This question appeared to me when I was thinking about General Relativity. During the expansion of space, the space in which particles reside is expanded. Now, general relativity states gravity is the ...
• 161
132 views

### Can the curvature of a closed universe be measured?

Suppose we were in a closed universe likened with a spherical surface such as that of the Earth's surface. Assume also that the limits, represented by the limits of the observable universe, ...
1 vote
104 views

### Coasting universe Hubble parameter behaviour

I’ve recently been trying to wrap my head around the potential behaviours of the Hubble parameter over time for various cosmological models but I’ve run into a little snag when it comes to a coasting ...
• 23
6k views

### How do we know we're not getting bigger?

Alright so I've been thinking a lot about how the universe expands and I've always wondered if we're getting bigger as well. Since everything would be getting larger at an equal rate (tools of ...
• 169
1 vote
32 views

### Gas heating due to high viscosity?

Has there been any observation where a gas or a fluid expands following the Hubble flow (the expansion of the universe) and gets heated because it has a large viscosity (transforming its kinetic ...
• 827
1 vote
90 views

### Expansion-Collapse cycle in cosmological structures?

Concerning cosmological structures (like galaxies, clusters of galaxies, gas bodies, superclusters...etc) if the elements that make them are close enough they will be attracted towards each other by ...
• 827
90 views

### Kinetic energy in cosmology?

Spacetime expands at an accelerated rate due to the Hubble flow. In many papers that I've read, objects coupled to the Hubble flow are treated as if they have some velocity and kinetic energy ...
• 827
53 views

### Can objects join the Hubble flow for a given amount of time and then somehow abandon it?

When two objects (e.g. two galaxies) are sufficiently far apart, they join the Hubble flow and they get further away from each other. Normally, this would last "forever" (until, from the ...
• 827
60 views

### Critical density and Hubble parameter

Is it possible for the Hubble parameter of an /expanding/ universe made of radiation, matter and cosmological constant to be increasing with time? I'm trying to figure out if any form of scale factor \$...
• 53
215 views

### Farthest distance two objects are "gravitationally bounded", considering expansion of the Universe

The narrative is: "on greater scales the expansion of the Universe dominates, but on smaller scales gravitationally bounded objects still stay bounded". But how small is meant by "...
• 647
71 views

### Limits of the universe

I am considering the limit in the expanding universe where the expansion matches the speed of light. If we consider that the expansion is accelerating, how will the situation be after the next 14 ...
205 views

### Does gravitational lensing disprove the cosmic expansion theory [closed]

Recently there was a picture from the JWST that showed a galaxy that was 12.5 billion light years away. Its light had been lensed by a galaxy that was 5 billion light years away. In a static universe ...
355 views

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

I got this bizarre idea in my mind, after reading SCP-3321 There, as a person gets teleported through a wormhole, ends up at the edge of the Observable Universe, 46 billion light-years away from Earth....
• 2,613
78 views

### Deviations of conservation laws in the context of cosmological evolution?

If energy is "not conserved" in General Relativity (or at least, it is difficult to define it) in the context of an accelerating expanding spacetime (like it happens in our Universe), are ...
• 827
829 views

### Why would galaxies (and galaxy clusters) be unaffected by the expansion of space?

"Gravity holds galaxies and cluster of galaxies together, and they get farther away from each other--without themselves changing in size--as the universe expands." Quoted from the OpenStax ...
• 539
69 views

### How do we know universe is expanding and not light slowing down? [duplicate]

We tend to take some things for granted, for example the light of speed is constant, but what if it isn't? How can we know that light is not slowing down at great distances for example, or that light ...
• 31
233 views

### Why is the Hubble parameter constant for an accelerating universe?

The Hubble parameter is thought to become constant and to attain a value of about 60 km/s per parsec. So every second a parsec of space grows with 60km. Is this consistent with an accelerated ...
118 views

### Distance of distant galaxies

Can two galaxies, one with a redshift of 7 and the other a redshift of 8 have the same angular distance (distance from the Earth during the emission of the observed photons)? If not why ? Example: ...
2k views

### Does a measuring stick with a size of a billion lightyears in intergalactic space keep the same length in expanding space?

Imagine a measuring stick (a rod or ruler) made of ordinary matter which has a length of a billion lightyears. The space surrounding the ruler expands. Will the ruler keep the same length by the ...
• 259
98 views

### Will dark energy always repulse observable/dark matter?

We know that the universe is expanding at an increasing accelerated rate, but the ratio of dark energy to observable matter remains constant as dark energy remains constant (please correct me on this)....
• 317