
Theory challenging Einstein's view on speed of light could soon be tested - antouank
https://www.theguardian.com/science/2016/nov/28/theory-challenging-einsteins-view-on-speed-of-light-could-soon-be-tested
======
CarolineW
So many submissions, and yet _no_ discussion! Is no one interested in this?

[https://news.ycombinator.com/item?id=13061826](https://news.ycombinator.com/item?id=13061826)

[https://news.ycombinator.com/item?id=13058356](https://news.ycombinator.com/item?id=13058356)
(contains link to the actual paper)

[https://news.ycombinator.com/item?id=13057823](https://news.ycombinator.com/item?id=13057823)

[https://news.ycombinator.com/item?id=13053305](https://news.ycombinator.com/item?id=13053305)

[https://news.ycombinator.com/item?id=13050196](https://news.ycombinator.com/item?id=13050196)

[https://news.ycombinator.com/item?id=13041033](https://news.ycombinator.com/item?id=13041033)

~~~
raattgift
I'd add [https://arxiv.org/abs/1603.03312](https://arxiv.org/abs/1603.03312)
too, as a link to the actual paper's arxiv version.

Bimetric theories are pretty strongly disfavoured on the grounds that there is
an apparent universal coupling to the single metric g_\alpha\beta which lets
us treat the metric as a geometrical property of spacetime, which gives us
General Relativity everywhere we can presently look, and even in places we
can't (GR as an effective field theory should hold up inside the horizons of
the largest black holes until fairly close to the centre of mass).

One can try for a UV completion of General Relativity in a variety of ways,
and broken symmetry approaches are fairly popular. In essence, an energy-
dependence in the speed of massless matter particles can be transformed into a
time-dependence, and with some care one can arrive at equal speeds for
massless matter waves and massless gravitational waves at some plausible point
in the early universe, with a cutoff that forbids observation of time-
dependent speed of light in the later universe (and in particular not in our
laboratories or in emissions from inspiralling black holes).

Cosmic Inflation also relies upon symmetry breaking at the end of the
inflationary epoch, but does so without changing the behaviour of the matter
fields; forbidding observable patches of inflation in the modern universe is a
requirement.

Partly consequentially, it seems perfectly reasonable to wonder whether a
bimetric theory could decay to General Relativity early after the big bang.
Rosen proposed a bimetric theory in 1940, for example, and it was only
effectively ruled out by indirect observations of loss of momentum from binary
pulsars (notably Hulse-Taylor), and has only just been killed off thanks to
the results from LIGO. Others have proposed bimetric theories since Rosen,
with varying ways of matching the observables that we have at present (since
all the observables are consistent with General Relativity).

A much higher temperature for the phase transition pushes observable effects
away from all but the highest-energy GW producers after galaxy formation, but
increases gravitational wave artifacts in the relic fields like the CMB and
its neutrino and possibly other equivalents. These artifacts are central to
the testability claim of the authors. Magueijo elsewhere (the authors' endnote
[7]) has argued that adding in modified dispersion relations also solves many
of the issues cosmic inflation does (spectral index, horizon problem).

Peter Coles highlights his own comment here:

[https://telescoper.wordpress.com/2016/11/28/nailing-
cosmolog...](https://telescoper.wordpress.com/2016/11/28/nailing-cosmological-
jelly-to-the-wall/)

It's hard to improve upon that. Observational support for Afshordi &
Magueijo's idea -- barring any theoretical problems they discover in further
elucidating it -- will only constrain cosmic inflationary models, rather than
kill them off. Conversely, observational evidence sufficient to kill off their
idea will not kill off bimetric gravity theories, it can only constrain them.

(One could argue that either way it's good science, since constraining the
space of viable theories is always useful.)

Finally, Teh Grauniad's article offers up this direct quote. I hope they got
it wrong: 'David Marsh, of the Centre for Theoretical Cosmology at Cambridge
University, is not giving up on inflation yet. “The predictions of inflation
developed by Stephen Hawking and others...' It is a serious oversight not to
lead off such a list with Alan Guth, even if he's not at Cambridge or even
British. [
[https://en.wikipedia.org/wiki/Alan_Guth](https://en.wikipedia.org/wiki/Alan_Guth)
]

ETA: finally, finally, another way of considering this idea is that in General
Relativity the geometry of spacetime induces the Poincaré invariance on the
tangent spaces (by inducing the Minkowski metric on them). That is how we get
the symmetries of Special Relativity (which are baked into the Standard Model)
at every point. A bimetric theory cannot induce this symmetry everywhere, and
so cannot be valid anywhere the Standard Model is. So bimetric approaches are
almost obliged to decay so early that before the decay is unobservable even in
principle. The paper here does the same, but with the idea that the relic
fields will hold a signature of the decay. By implication, the relic fields
should also hold a signature of Beyond-the-Standard-Model physics, and maybe
ultra high energy astrophysical processes in nature ought to reveal such BTSM
physics too.

