Acceleration/deceleration of the expanding universe?

In summary: So if at time zero it was one meter long and at time t it had expanded to two meters long, then a equals 2.The curve shows how the scale factor (which describes the size of the universe) has changed over time. It is a plot of the value of a for each point in time. So if the scale factor is 2 at time t=0.5, and 3 at time t=1, the curve will pass through (0.5,2) and (1,3).The scale factor is a function of time and is often written with a subscript a(t) so it
  • #1
PeterZimmerman
1
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Apparently the generaly accepted interpretation is that the expansion is speeding up, mainly by observing the redgarbageing of supernovae of distant galaxies and establishing their distance and redshifting indicating the speed at which they are moving away from us.

The further away the faster they are moving according to observations.

This as I understand it was seen as proof of an acceleration.

Which in turn was explained by the invention of dark energy to counter the gravitational force and accelerate the expansion which would otherwise have been decelerated and eventualy possibly reversed if the mass of the universe was great enough to counter the force of the initial "explosion" which would lead to a big crunch.

For example a galaxy 13 billion Ly away would be observed moving away from us 3 times faster than a galaxy 1 billion light years from us. (no exact numbers just the basic idea of how it was explained)

While the explanation of darkenergy propelling the universe towards a Rip or fading seems like it could be possible, I'm thinking it sounds like the explanation has become a theory of too much.


Couldn't it be that the galaxy's speed observed 13 billion years back could have been faster simply because all matter in the universe was moving faster 13 billion years back before gravity had slowed it all down and decelerated the expansion.

Which by looking at closer galaxies would suggest that the universe is still expanding but not at the same speed it was 13 Billion years ago.
But then it would indicate a deceleration of the expansion.
Then you wouldn't need dark energy or at the very least far less of it.

After all the galaxies 13 billion lightyears away were 13 billion lightyears away and moving at the now observed speed 13 billion years in the past, their current state is unknown but calculations and estimates could be atempted I guess. But basicly it seems we're seeing their speed 13 billion years back while closer galaxys at 1 billion years are recorded at their speeds 1 billion years ago.

The observed speed/distance spread seems fairly homogenous aswell. The age of the data collected seems to be what determines of their expected speed in the expansion of the universe.
It's also possible that small deviations of the mass of the observed supernovae could interfere with the data a little even if's generaly a reliable "standard light".

The idea that the universe is currently expanding seems reasonable enough, but the idea that it's accelerating seems less intuitive.

Maybe I missed some of the "proof" or maybe it was overly simplified. I've tried to keep it short by not going too much into the expansion of space driven by dark energy and all that. Since my concern isn't the the idea itself but rather that I think it might be, like I said, a theory of too much. a little bit like Einstein tried to create a constant to keep the universe in a static state. Which he later considered one of his biggest misstakes.

Maybe it's in the interpretation of the observations. Maybe someone could explain the proof provided by the supernova standard lights which tells us the universe is expanding at an accelerated pace.
As I understand it one can theoreticaly get the distance by measuring the luminosity of the standard light type 1a supernova, and it's speed relative to us from it's redshifting, the acceleration part seems unclear though.
 
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  • #3
Hi Peter,
What is your source of information? I ask because that will help posters here get a better perspective before responding. Your questions may not be so easily/briefly answered. It could take a book or two!

COSMOLOGICAL EXPANSION...a one paragraph synopsis

A short answer is that using a particular cosmological model, the FLRW model and the conventions and assumptions that go into it, we calculate ‘superluminal’ (faster than light] expansion both at really early cosmological times and continuing to today at really great distances. We cannot directly observe such rapid expansion [not because the light will never reach us, although that happens in some circumstances, but because such distances are measured [calculated] along a curve of constant [fixed] cosmological time. This is analogous to instantaneously reading a measure on a fixed ruler.

Another insight which helped me is this :

The classic thought experiment used is the “tethered galaxy” problem (Harrison 1995). In this, a test galaxy in an expanding universe is held at rest with respect to the origin at a cosmological distance. By Hubble’s law, we would expect this galaxy to be receding, however we prevent this, artificially holding the test galaxy in place. The question is, when the galaxy is released, what does it do? In fact, what the particle does once being released depends on the acceleration of the universe. If the scale factor is accelerating the particle moves away but if it is decelerating the particle moves towards the origin [see Barnes et al. (2006) for the full details].

It is currently believed the scale factor [a(t)} is 'accelerating'. But it is slower currently than it has been over cosmological history.

One of my very favorite discussions in these forums is here:
[Up through the first 100 posts or so]

Expanding Universe: Does Space Expand
[Wallace]
https://www.physicsforums.com/showthread.php?t=162727&highlight=current+flow&page=2


Try this "frequently asked questions" which interest you here:

[It is a reasonably good place to start]

http://www.astro.ucla.edu/~wright/cosmolog.htm

Another is this article:

Misconceptions about the Big Bang: Scientific American 6/14/08 6:50 PM
http://www.sciam.com/article.cfm?id=misconceptions-about-the-2005-03 [Broken]


A more advanced discussion was referenced in these forums in another discussion...
Don't read this one first.

http://arxiv.org/pdf/1304.4446v1.pdf
 
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  • #4
An illustration of how the scale factor, the relative expansion of space, has varied over time via different
cosmological parameters is here:

http://www.astro.ucla.edu/~wright/cosmo_03.htm

[This is in the is ned wright tutorial I posted previously]

We are at time ZERO in the plot. Our universe is approximated via the MAGENTA [pink]
curve...but those factors are now slightly out of date...

Here is a nice synopsis of the scale factor, from Marcus of these forums:

The scale factor curve a(t) is the expansion history. It is a picture of how the scale factor has increased over time. The slope of that curve, at any given time, is the “speed of expansion” at that time. It started off very steep [high expansion], leveled off slightly and is now increasing gradually [in our energy dominated cosmological phase]. It has always been positive slope---a(t) has always been climbing---but the slope has varied...
 
  • #5


I can provide some insight and clarification on the topic of the acceleration and deceleration of the expanding universe.

Firstly, the idea that the expansion of the universe is accelerating is indeed the generally accepted interpretation based on observations of distant supernovae and their redshift. This was first proposed in 1998 by two independent research teams, who both found evidence that the expansion of the universe is accelerating.

The explanation for this acceleration is the concept of dark energy, which is a hypothetical form of energy that is thought to permeate all of space and counteract the gravitational pull of matter. Without dark energy, the expansion of the universe would be expected to slow down and possibly even reverse, leading to a "big crunch" scenario. However, the observations of distant supernovae indicate that the expansion is actually speeding up, suggesting the presence of this mysterious dark energy.

It is important to note that the observations of distant supernovae are not the only evidence for an accelerating universe. Other measurements, such as the cosmic microwave background radiation and the distribution of galaxies, also support this idea. Additionally, the concept of dark energy has been further supported by independent observations, such as the growth rate of galaxy clusters and the large-scale structure of the universe.

In regards to the concern that the acceleration of the universe may be a "theory of too much," it is worth noting that scientists are constantly questioning and refining our understanding of the universe. The concept of dark energy is still a subject of ongoing research and debate, and alternative theories have been proposed to explain the acceleration of the universe. However, at this point in time, the evidence for an accelerating universe is strong and supports the idea of dark energy.

Regarding your suggestion that the observed speed of distant galaxies may have been faster in the past due to a lack of gravitational pull, this idea has been considered and ruled out through further observations and calculations. The observations of distant supernovae are not just a snapshot of their current state, but also take into account the time it takes for light to travel to us. This means that we are seeing these galaxies as they were in the past, and the data collected has been carefully analyzed to account for any potential deviations in the mass of the supernovae.

In conclusion, while there may be ongoing debates and alternative theories, the current evidence strongly supports the idea of an accelerating expansion of the universe driven by dark energy. As scientists, it is important for us to continue questioning and refining
 

1. What is the expansion of the universe?

The expansion of the universe is the theory that states that the space between galaxies is continuously expanding. This means that the distance between galaxies is increasing over time.

2. What is acceleration/deceleration of the expanding universe?

The acceleration/deceleration of the expanding universe refers to the rate at which the expansion is occurring. If the rate of expansion is increasing over time, it is known as acceleration. If the rate of expansion is decreasing, it is known as deceleration.

3. How is the acceleration/deceleration of the expanding universe measured?

The acceleration/deceleration of the expanding universe is measured using a variety of methods, including the observation of the redshift of light from distant galaxies, the study of the cosmic microwave background radiation, and the analysis of Type Ia supernovae. These methods allow scientists to track the expansion and determine if it is accelerating or decelerating.

4. What is causing the acceleration/deceleration of the expanding universe?

The cause of the acceleration/deceleration of the expanding universe is currently unknown. Some theories suggest that dark energy, a mysterious force that makes up about 70% of the universe, is responsible for the acceleration. Other theories propose modifications to the theory of gravity to explain the acceleration.

5. How does the acceleration/deceleration of the expanding universe affect us?

The acceleration/deceleration of the expanding universe has a significant impact on the fate of the universe. If the expansion continues to accelerate, it is likely that the universe will continue expanding indefinitely and eventually result in a "heat death" with all matter becoming too far apart to interact. If the expansion decelerates, it is possible that the universe will eventually collapse in on itself in a "big crunch." However, the exact outcome is still uncertain and is an active area of research in cosmology.

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