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Relativity and Cosmology

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Heavyweights Flexing BICEP Muscles

Posted November 30, 2014 6:00 PM by Jorrie
Pathfinder Tags: cosmology Cyclic model inflation

I posted a Blog entry in Jan 2011 on the Colliding Branes Cosmic Model, in which the ideas of Neil Turok and Paul Steinhardt were discussed. Their model predicted the same things as in standard inflationary cosmology, except for the absence of gravitational wave (GW) effects in the Cosmic Microwave Background (CMB). The inflation paradigm predicts quite strong primordial gravitational waves. This is supposed to be the main discriminating factor, with the inflationary scenario pictured here.

For a clearer image, see the source in Wikipedia

The Planck mission was expected to shed some light on the issue, but its first data analysis did not show up any such evidence. First round tied, because it did not rule out primordial GWs (at below observable threshold) either, but this has obviously fueled the issue of the colliding branes theory - could it possibly be a confirmation?

In the meantime, the Background Imaging of Cosmic Extragalactic Polarization (BICEP2) telescope, operated at the South pole, specifically searched and reported that the elusive GWs have been found. This seemed to be the end of the Colliding Branes theory, until some doubts were raised by cosmologists and astronomers, some of them not involved in or even supporting the Colliding Branes idea. The photo below shows the BICEP2 system at the South Pole (also Wikipedia).

The problem seems to be that the signal could be weaker than the equivalent signals coming from dust in our own Galaxy. Now a further Planck data release appears to confirm the galactic dust interpretation and it may refute the BICEP2 interpretation of the signal. Headlines changed from something like "Inflation theory confirmed" to e.g. "BICEP2 gravitational wave result bites the dust thanks to new Planck data". No firm conclusion is reached yet and cooperation between the Planck and BICEP2 teams is now on the cards.

Lately, heavyweights like Brian Greene, Alan Guth, Andre Linde, Paul Steinhart and others are "taking off the gloves" and are very vocal in the support of their favorite theories. I have just watched a fascinating discussion led by Brian Greene with the other three plus Amber Miller (Columbia Uni) and John Kovac (BICEP2), where Steinhardt was the lone voice for the Colliding Branes (now simply called the Cyclic Cosmic Model).

Both Inflation with its "multiverse" and the Cyclic model with its "bouncing branes" have some issues and have undergone some changes over the last five years or so. If you just want a summary of the issues between the models, watch the last ten minutes or so of this rather lengthy video, from around time 1:24:00, where Greene asked Steinhardt to summarize his view. Very interesting responses from Guth and Linde followed. Enjoy and then let's discuss it from an engineering POV (if such a view is at all possible in modern cosmology!)

I still do not quite understand the workings of the contracting phase that follows the present accelerating expansion, but since it seems somewhat promising, I am willing to learn more. I quite liked Steinhardt's talk: What we have learned form BICEP2.

Or, if you are keenly interested, try his "The Cyclic Theory of the Universe" (student edition), a quite technical 48-pager saying most of what there is to say about this theory.



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Re: Heavyweights flexing BICEP muscles

12/01/2014 8:03 AM

Thanks for the links. I am still digesting the material.

There are so many competing ideas. Remarkably, the Standard Model still holds up pretty well.

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