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View Full Version : New supernova analysis reframes dark energy debate



Gigabyte
2017-Oct-14, 04:19 AM
The accelerating expansion of the Universe may not be real, but could just be an apparent effect, according to new research published in the journal Monthly Notices of the Royal Astronomical Society (https://academic.oup.com/mnras/).


https://www.ras.org.uk/news-and-press/3046-new-supernova-analysis-reframes-dark-energy-debate

antoniseb
2017-Oct-14, 01:18 PM
I keep seeing press releases from this same New Zeeland group saying more or less the same thing with no new information. The article you link to discuses the upcoming Euclid probe, which should be able to remove enough uncertainty to be clearer about this, but it is worth noting that the SN 1a data is one path of analysis that supports Dark Energy, but it is not the only one, but it is the only one that this group is trying to cast doubt on.

Gigabyte
2017-Oct-14, 02:09 PM
Helpful links

https://www.nature.com/articles/srep35596?WT.feed_name=subjects_astronomy-and-astrophysics

https://www.researchgate.net/publication/317820235_Apparent_cosmic_acceleration_from_type_I a_supernovae

Shaula
2017-Oct-14, 03:56 PM
I keep seeing press releases from this same New Zeeland group saying more or less the same thing with no new information. The article you link to discuses the upcoming Euclid probe, which should be able to remove enough uncertainty to be clearer about this, but it is worth noting that the SN 1a data is one path of analysis that supports Dark Energy, but it is not the only one, but it is the only one that this group is trying to cast doubt on.
It is an interesting theory, however it has some significant issues that they have not addressed so far. The first is that while it still needs dark matter to match the WMAP data it needs a lot of baryonic dark matter. Enough to seriously affect nucleosynthesis (which wouldn't work as we currently understand it with the mix they need) and to have some significant, unobserved, structure effects. For example it needs a very large (3-4x the visible mass of the Milky Way) dust cloud with some interesting properties and some very challenging stability issues to get rotation curves to work.

It is worth mentioning that some of these issues might go away with more detailed analysis, they are estimates as things stand (but reasonably good ones).

It is an interesting model and it is always worth having people developing and testing alternatives to the current mainstream. But it has as many failures as successes, so it is early to get carried away with it.

Ken G
2017-Oct-14, 06:29 PM
Yes, I recall the objections to this were raised on another thread: they used as their constraint only one set of observations-- the type Ia supernovae. But the consensus cosmology uses three main contraints, the Ia, the CMB, and models of Big Bang nucleosynthesis. It is well known that when you have uncertainty in your data, if you focus on only one dataset you might find solutions that work as well or better than solutions that must respect all three constraints. I think that's all that's happening here-- the Ia data alone simply cannot force us to accept dark energy, but it is hard to avoid it if we must satisfy all the existing observations. In short, this group is playing on an unfair field, since they only feel compelled to fit one set of data.

Gigabyte
2017-Oct-14, 06:43 PM
In standard cosmology, we assume a background expanding as if there were no cosmic structures. We then do computer simulations using only Newton’s 330-year old theory. This produces a structure resembling the observed cosmic web in a reasonably compelling fashion. But it requires including dark energy and dark matter as ingredients.Even after inventing 95 percent of the energy density of the universe to make things work, the model itself still faces problems that range from tensions to anomalies (https://doi.org/10.1142/S021827181630007X). It's an interesting problem.

To avoid stagnation and nurture a vibrant scientific culture, a research frontier should always maintain at least two ways of interpreting data so that new experiments will aim to select the correct one. A healthy dialogue between different points of view should be fostered through conferences that discuss conceptual issues and not just experimental results and phenomenology, as often is the case currently. http://www.newsweek.com/dark-energy-exist-einstein-general-relativity-630260


"The evidence for accelerated expansion is, at most, what physicists call '3 sigma.' This is far short of the 5-sigma standard required to claim a discovery of fundamental significance," co-author Subir Sarkar, of Oxford University in England, said in a statement (http://www.ox.ac.uk/news/science-blog/universe-expanding-accelerating-rate-%E2%80%93-or-it).

"So it is quite possible that we are being misled, and that the apparent manifestation of dark energy is a consequence of analyzing the data in an oversimplified theoretical model — one that was in fact constructed in the 1930s, long before there was any real data," Sarkar added.
https://www.space.com/34503-universe-expansion-accelerating-dark-energy.html

Ken G
2017-Oct-14, 06:50 PM
Yes, I think there is benefit in fostering alternate conceptual approaches-- there is no more central requirement for good science than the avoidance of dogma.

Gigabyte
2017-Oct-14, 07:10 PM
While dark matter is rather easy to dismiss (just find the matter that we can't detect), the dark energy matter is another ball of string theory altogether. I may have mixed metaphors there, but the Universe starting to speed up (for some unknown reason, by some unknown mechanism, utilizing an unknown form of energy) is hard to swallow. Considering how many new and unexpected things keep getting discovered, the matter is certainly up for discussion. Or rather the existence or non-existence of the dark energy.

01101001
2017-Oct-15, 02:14 AM
I may have mixed metaphors there, but the Universe starting to speed up (for some unknown reason, by some unknown mechanism, utilizing an unknown form of energy) is hard to swallow.

It's not hard to swallow, or, more appropriately, accept as fact. Maybe it's hard for you to swallow, but consider that is you and not it.

I figure: once you accept the preposterous existence of the Universe, its peculiar qualities just come as part of the package.

trinitree88
2017-Oct-18, 03:05 AM
Hmm. "far short of a 5 sigma discovery necessary to claim a discovery of fundamental importance" I guess some are more equal than others. pete

Gigabyte
2017-Oct-18, 03:34 PM
In short, this group is playing on an unfair field, since they only feel compelled to fit one set of data. I don't know enough to argue either way. I just find it interesting. The reasoning for the CBM supporting dark energy seems to be it results in less structure. Dark energy would keep the Universe from being structured.


In the second paper Das, along with Blake Sherwin of Princeton University and Joanna Dunkley of Oxford University, looks at how lensing could reveal dark energy. Dark energy acts to counter the emergence of structures within the universe. A universe with no dark energy would have a lot of structure.
http://physicsworld.com/cws/article/news/2011/jul/15/dark-energy-spotted-in-the-cosmic-microwave-background

I'm probably wrong, but recent studies seem to show the Universe does have a lot of structures, in fact it looks like one huge structure.

https://www.universetoday.com/137446/astronomers-find-missing-normal-matter-universe-still-looking-dark-matter-though1/

Most of this is way beyond my pay grade.

Reality Check
2017-Oct-18, 09:52 PM
The way that CMB data usually shows us that the majority of the universe is dark energy (https://en.wikipedia.org/wiki/Dark_energy#Evidence_of_existence) is that we take cosmological models + other physics and predict what the CMB properties would be with a mix of baryonic matter, non-baryonic matter and dark energy. There is no way to match the data if we put the amount of dark energy to zero. The best parameters that fit the data gives the same amount of dark energy as we get from Type 1a supernovae. But this is an indirect method.

Dark energy spotted in the cosmic microwave background (http://physicsworld.com/cws/article/news/2011/jul/15/dark-energy-spotted-in-the-cosmic-microwave-background) was "new direct evidence for dark energy" (in 2011). Dark energy influences the amount of structure. No dark energy gives a lot of structure. The observed amount of dark energy gives a little less structure. If we pump up dark energy to a hypothetical enormous level then we could have no structure. A key point of the second paper is that lensing of the CMB by structures shows that dark energy is not zero and it matches other measurements.