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I have been recently researching the topic of the expansion of the Universe. Specifically, the 1998 discovery by Saul Perlmutter that claims the Universe is expanding at an accelerated pace. From my understanding, the premise of this discovery was that the observed redshift of a Supernova 7 Billion Light years away was perceived to be receding at a slower rate than that of the redshift observed of a Supernova 1 Billion Light Years away. The conclusion was that the Universe was expanding at a slower rate 7 Billion years ago than it was 1 Billion years ago, thus an acceleration of expansion.

I also understand that two other teams had similar results based off different studies, Erick Guerra and Ruth Daly (Studied 14 radio galaxies indicating the mass of the Universe is 20% smaller than would be necessary to stop the expansion of the universe and cause a collapse back unto itself) and, Neta Bahcall and Xiaohui Fan (Studied the mass density of galaxy clusters and came to a similar conclusion).

I am curious as to what your thoughts are on the conclusion of this data. Specifically, do you have other theories as to why the accelerations of the two Supernovae were perceived to be receding at different rates? What else could cause our perception to show the varying expansion rates? 

As a preface to my thoughts on the matter, I should state that I have not been researching this for long and have no background in the field, just a curious and wandering mind.

Anyway, I was wondering if the differing space environments at the time of the stars' destruction have an effect? E.g. The Universe was more dense at the time of the Supernova 7 Billion years ago than that at the time of the Supernova 1 Billion years ago. That being the case, 7 Billion years ago, when the Universe was expanding, the density of the Universe, in my mind, could have impeded the acceleration of expansion more so than the less dense Universe that the Supernova 1 Billion years ago was experiencing. This leads me to believe that the acceleration would be naturally increasing as the density continues to decrease over time.

Another question I have been pondering is the point of reference for the two differing Supernovae, which is Earth. Our Galaxy is also caught in the expansion of the Universe, so the rate at which we are moving would also have to be factored in. As well as the locations and directions of all three points of reference, Earth, Supernova 1, and Supernova 2. If Our Galaxy is expanding in a similar direction through space as that of Supernova 1 (7 Billion Years ago) and an opposite direction than that of Supernova 2 (1 Billion Years ago), then the obvious outcome would be that Supernova 2 would appear to be receding at a more accelerated rate than Supernova 1 due to the variance of our location in space in comparison to that of the observed points.

From my understanding, which is very minuscule, redshift does not allow the observer to determine the direction of expansion, just the speed and distance. I suppose given the variables, one can calculate the direction of expansion if given enough time to observe the Supernovae, but seeing as the observer only has approximately 18 days before a type 1a Supernovas light fades, that does not seem likely to me.

Any feedback is appreciated. Even if it is to tell me I am wrong. I enjoy hearing all perspectives.

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    $\begingroup$ What's the question? $\endgroup$
    – Yashas
    Commented Mar 12, 2017 at 4:03

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You probably can learn more if you read a little more on cosmology, general relativity, and the expansion of the universe.

For each of your points, all of those have been considered and accounted for.

  • the different densities 1 and 7 billion years ago? It's been known, it's known how the densities evolve with time, and was accounted for in estimating if the velocity should be more or less. Yes, the densities of matter and normal energy are less now than in the past because we have more space created by the expansion and the redshifts make photons lose energy. But it's accounted for in the calculations, and it still means that based on the amount of matter and normal energy we should be decelerating. The fact that we measure it as being that we are accelerating means we are, and actually also that there is something we have not accounted for, that causes the acceleration. That is what we call dark energy. The latest measurements and calculations made from the cosmic microwave background has it that dark energy is now about 70% of all the matter energy in the universe, that it's been increasing and we've been accelerating the last 8 billion years. Before that we've measured that we were decelerating, when the dark energy was much less and the attractive nature of matter and normal energy causes us to decelerate.

  • the different directions of the two supernovas? It makes no difference, there's been many more measurements done all around, and also the redshift measurements. They are all receding with us in what is called the cosmic flow. The direction is 'away'. Like two dots painted on a balloon that is inflating, or 17 dots, they all will recede from each other. The redshifts measured are consistent with the supernovas distances observed. It's been observed that at those cosmological distances the universe's density, averaged over about 50-100 light years, are the same at the same distances in all directions, the universe is homogeneous and isotropic in the large.

  • Redshift and direction? You are right we measure if away (redshift) or towards (blueshift) us. Direction for us to look to the NE from us, or SE or N makes no difference. The universe looks the same to us in all directions, in the large. What of our peculiar velocity? Yes, earth moves around the sun, which goes around the Galaxy, which moves WRT outer cluster and with our galaxy cluster. It's a few hundred Kms/sec (I forget the exact number), and yes we have to subract it out of our observations. When we do we are then in a reference frame that is moving with the average cosmological flow. No problems with the people who did that math, there's been much harder issues.

So, none of that is an issue as understood in our model of the universe. See a wiki article about our expansion at https://en.m.wikipedia.org/wiki/Metric_expansion_of_space.

See accelerated expansion at https://en.m.wikipedia.org/wiki/Accelerating_expansion_of_the_universe

The real issue is we still don't know what exactly is the dark energy. We think it is the vacuum energy of spacetime, but can't understand why it is the value it is. Still a lot of research in that area, and also dark matter.

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  • $\begingroup$ Thank you for your response and for providing the links. Great information. I will definitely do more research on Cosmology, General Relativity and the Expansion of the Universe. I have also been doing a little research on Dark Energy and Dark Matter, which seem to have more questions than answers. There is still a lot more for me to learn. Like I said, I have only recently began researching these topics but am very fascinated by them and enjoy the knowledge. $\endgroup$
    – Variable
    Commented Mar 12, 2017 at 7:03
  • $\begingroup$ Regarding the Dark Energy making up 70% of the matter energy and increasing, is it possible that dark energy could multiply by self replication through a process similar to that of binary fission in bacteria? I suppose it will be impossible to know without first being able to detect the dark energy, but without a known source of the increased dark energy, self replication seems to be a plausible cause of the increasing levels. simple.wikipedia.org/wiki/Binary_fission $\endgroup$
    – Variable
    Commented Mar 12, 2017 at 7:30
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    $\begingroup$ Dark energy is the worse unknown we have. The fact that it makes up 70% of the universe is semi-embarrassing. The thought is rather that it is the energy density of vacuum, and as space expands it forms more vacuum and thus more energy. But we can't figure out why it is a certain number, the measured value. And have not been able to detect it directly, like have it directly interact is some accelerometer or other ways of detection. Pretty mysterious still. I think the self replication with space expanding feels ok, but why it's other than 0 or a very large number. Yes, detect it first $\endgroup$
    – Bob Bee
    Commented Mar 13, 2017 at 2:26
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    $\begingroup$ Or some general relativity doubters say modify general relativity to include some other fields other than the spin 2 tensor, and it may explain that. Some calculations have been made and it is possible, but the predictions so far with that are also inconsistent with more normal measurements of general relativity in the solar system like light deflection. So they are trying some ways of screening that other field at shorter distances like the solar system, really just trying different things to see if anything works. But detection would make the biggest difference. $\endgroup$
    – Bob Bee
    Commented Mar 13, 2017 at 2:30

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