RAS: New Supernova Analysis Reframes Dark Energy Debate

[bystander]

New Supernova Analysis Reframes Dark Energy Debate
Royal Astronomical Society | 2017 Sep 14

A computer-simulated image depicting one possible scenario of how light sources are distributed in the cosmic web. Credit: Andrew Pontzen and Fabio Governato / UCL

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The accelerating expansion of the universe may not be real, but could just be an apparent effect ... The new study ... finds the fit of Type Ia supernovae to a model universe with no dark energy to be very slightly better than the fit to the standard dark energy model.

Dark energy is usually assumed to form roughly 70% of the present material content of the universe. However, this mysterious quantity is essentially a place-holder for unknown physics.

Current models of the universe require this dark energy term to explain the observed acceleration in the rate at which the universe is expanding. Scientists base this conclusion on measurements of the distances to supernova explosions in distant galaxies, which appear to be farther away than they should be if the universe’s expansion were not accelerating.

However, just how statistically significant this signature of cosmic acceleration is has been hotly debated in the past year. The previous debate pitted the standard Lambda Cold Dark Matter (Lambda CDM) cosmology against an empty universe whose expansion neither accelerates nor decelerates. ...

Rather than comparing the standard Lambda CDM cosmological model with an empty universe, the new study compares the fit of supernova data in Lambda CDM to a different model, called the ‘timescape cosmology.’ This has no dark energy. Instead, clocks carried by observers in galaxies differ from the clock that best describes average expansion once the lumpiness of structure in the universe becomes significant. Whether or not one infers accelerating expansion then depends crucially on the clock used.

The timescape cosmology was found to give a slightly better fit to the largest supernova data catalogue than the Lambda CDM cosmology. ...

Apparent Cosmic Acceleration from Type Ia Supernovae - Lawrence H. Dam, Asta Heinesen, David L. Wiltshire

• Monthly Notices of the RAS 472(1):835 (Nov 2017) DOI: 10.1093/mnras/stx1858
  arXiv.org > astro-ph > arXiv:1706.07236 > 22 Jun 2017 (v1), 13 Sep 2017 (v2)

[MargaritaMc]

Doing a search for "timescape cosmology" hoping to get a better grasp on what is being proposed, I found this article by David Wiltshire in The Conversation.

It begins:

Can we ditch dark energy by better understanding general relativity?
https://theconversation.com/can-we-ditc ... vity-76777
June 29, 2017 5.47am BST

https://thewire.in/152793/dark-energy-g ... y-physics/
BY DAVID WILTSHIRE AND ALAN COLEY 

If dark energy is just a grand illusion – as some suggest – then rethinking our interpretation of the basic principles of general relativity in a complex universe is crucial.

A renewed suggestion that dark energy may not be real — dispensing with 70% of the stuff in the universe — has reignited a longstanding debate.

Dark energy and dark matter are theoretical inventions that explain observations we cannot otherwise understand.

On the scale of galaxies, gravity appears to be stronger than we can account for using only particles that are able to emit light. So we add dark matter particles as 25% of the mass-energy of the Universe. Such particles have never been directly detected.

On the larger scales on which the Universe is expanding, gravity appears weaker than expected in a universe containing only particles – whether ordinary or dark matter. So we add “dark energy”: a weak anti-gravity force that acts independently of matter.

[MargaritaMc]

Background from 2007. This article from the Australian Broadcasting Corporation explains what David Wiltshire is proposing as only Australians can:

Dark energy a furphy, says new paper
http://www.abc.net.au/science/articles/ ... 124258.htm
Friday, 21 December 2007 Marilyn Head
ABC

(BTW: Wikipedia tells me that A furphy is Australian slang for an erroneous or improbable story that is claimed to be factual. )

[MargaritaMc]

And yet more background (I'm really finding this fascinating.)

DAVID L. WILTSHIRE, Int. J. Mod. Phys. D, 18, 2121 (2009). 
https://doi.org/10.1142/S0218271809016193
FROM TIME TO TIMESCAPE — EINSTEIN'S UNFINISHED REVOLUTION
[...]In a universe dominated by voids of the size observed in large-scale structure surveys, the density contrasts of expanding regions are strong enough that a relative deceleration of the background between voids and the environment of galaxies, typically of order 10^−10 ms^−2, must be accounted for. As a result one finds a universe whose present age varies by billions of years according to the position of the observer: a timescape. This model universe is observationally viable: it passes three critical independent tests, and makes additional predictions. Dark energy is revealed as a mis-identification of gravitational energy gradients and the resulting variance in clock rates. Understanding the biggest mystery in cosmology therefore involves a paradigm shift, but in an unexpected direction: the conceptual understanding of time and energy in Einstein's own theory is incomplete.

[bystander]

Here you go:

arXiv: Timescape Cosmology
google: Timescape Cosmology

[MargaritaMc]

Here you go:

arXiv: Timescape Cosmology
google: Timescape Cosmology

Many thanks. I found the sources I've posted via a Google search on Timescape Cosmology, but had not thought of doing a specific arXiv search.
The piece in The Conversation is reprinted in The Wire, which is returned on the first page of the search.

edit
Oooh - just found this article, written yesterday (by clicking bystander's Google search...)

https://www.sciencealert.com/this-new-m ... st-as-well
This Surprising New Model of The Universe Doesn't Need Dark Energy at All
What's going on?!
MIKE MCRAE
15 SEP 2017

[Ann]

Interesting indeed. But I'll wait for various experts to look at this hypothesis and butt heads over it before I decide where I stand.

Ann

[MargaritaMc]

As you say, Ann, interesting, and we await further discussion by experts with even more interest. I don't think this piece, from October 2016, got posted here. I've done an advanced Google search and can't find it.

https://www.nature.com/articles/srep35596
Marginal evidence for cosmic acceleration from Type Ia supernovae

J. T. Nielsen, A. Guffanti & S. Sarkar
Abstract
The ‘standard’ model of cosmology is founded on the basis that the expansion rate of the universe is accelerating at present — as was inferred originally from the Hubble diagram of Type Ia supernovae. There exists now a much bigger database of supernovae so we can perform rigorous statistical tests to check whether these ‘standardisable candles’ indeed indicate cosmic acceleration. Taking account of the empirical procedure by which corrections are made to their absolute magnitudes to allow for the varying shape of the light curve and extinction by dust, we find, rather surprisingly, that the data are still quite consistent with a constant rate of expansion.

Affiliations of the authors
Niels Bohr Institute, University of Copenhagen, Blegdamsvej 17, Copenhagen 2100, Denmark
J. T. Nielsen & S. Sarkar
Dipartimento di Fisica, Università degli Studi di Torino, via P. Giuria 1, I-10125 Torino, Italy
A. Guffanti
Rudolf Peierls Centre for Theoretical Physics, University of Oxford, 1 Keble Road, Oxford OX1 3NP, UK
S. Sarkar

As the sciencealert article (that I added to my earlier post above https://www.sciencealert.com/this-new-m ... st-as-well) concludes

The team of researchers applied the timescape model to a large dataset of supernovae, and found it matched the changes in light-waves and luminosity with a likelihood that was indistinguishable from the ΛCDM model cosmologists currently favour.

None of this should be interpreted as a call to pack up shop and put dark energy into the filing cabinet.

For one thing, there would need to be a lot more evidence to convince the field to abandon one model for another, especially given there would be more at stake than just the question of dark energy.

But contrasting models can be a good way to make sure existing assumptions are reasonable and conclusions are watertight, while highlighting weaknesses that could potentially lead to improved solutions.

Dark energy isn't going anywhere soon. Time is ticking, however, and the day will come when we'll have a better idea of what lies in its shadows, if anything at all.