Friday, 4 January 2013

Phantom Energy and the Cosmic Horizon: Rh is still not a horizon!

Back to work, and a good start to the New Year. I will write more on this shortly, but our paper in Nature was published yesterday (the various astronomical societies I spoke to last year know about this result, but I've kept it quiet in professional circles - hopefully it will cause a few wave).

I know I keep saying it, but time is squeezed so a brief post today. And that's on another paper that I had accepted yesterday (I am still trying to catch up on the Christmas list... Ah well, I will get to those eventually).

I've written before about the work I have done on "The Cosmic Horizon". It's a long story, but there have been claims that this is a magical part of the Universe which has not been properly understood and modern cosmology has it all wrong etc. Simply put, these claims are wrong, but they keep appearing.

So, this new paper was in response to another paper which claimed our previous paper was wrong. As ever, the claims are wrong, as I describe.

As I've stated before, horizons are important things in cosmology, and there are some classic papers which describe their importance. There is another important quantity in cosmology, called the Hubble Sphere. Objects on the Hubble Sphere are moving away from us at the speed of light; to the rival papers, the Hubble Sphere is the mysterious "Cosmic Horizon".

There has been huge confusion over the years about what you can see in the Universe, and how can you observe things moving faster than the speed of light etc. If you find yourself in this state, I suggest you read this.

To cut to the chase, the claim is that the Hubble Sphere is a horizon, and that the light rays we receive are limited by the presence of the horizon (i.e. they can only go through it in one direction etc). In this paper, I showed that it is simple to make the Hubble Sphere increase and decrease by just mucking about with the constituents of the Universe, changing the spots of dark energy.

A picture speaks a thousand words:
The blue is the Hubble sphere, which initially moves away from us after the Big Bang, then, as phantom energy takes over, moves towards us, and then, as I allow the phantom energy decay into photons, it moves away from us again.

The red lines are light rays which, after the Big Bang, move away from us, then some turn around and arrive back as the observer. Notices that one light ray, the one that arrives at the observer at a time of 130 billion years after the Big Bang, crosses the blue line three times, in differing directions. This is not Horizon-like behaviour.

In fact, in mucking about with the make up of the Universe, I could make the blue line dance in and out, and play with a photon, making it cross the blue line as many times as I wanted before we observed it. Simply put, the blue line is well understood. It's the Hubble Sphere. It is not a magical Cosmic Horizon.

And just to make Prajwal smile, I did the calculations in Python.

Well done ... errrm .... me :)

Phantom Energy and the Cosmic Horizon: Rh is still not a horizon!

Geraint F. Lewis
There has been a recent spate of papers on the Cosmic Horizon, an apparently fundamental, although unrecognised, property of the universe. The misunderstanding of this horizon, it is claimed, demonstrates that our determination of the cosmological makeup of the universe is incorrect, although several papers have pointed out key flaws in these arguments. Here, we identify additional flaws in the most recent claims of the properties of the Cosmic Horizon in the presence of phantom energy, simply demonstrating that it does not act as a horizon, and that its limiting of our view of the universe is a trivial statement.

6 comments:

  1. I don't understand the cosmology, but I had two comments.

    * Thanks for the acknowledgement. :)
    * Prajwal's website is awesome.

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  2. Part of my webpage was made when I was staying in Hollywood, seriously! Thanks Brendon and Geraint :)

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  3. Geraint, you covered this very neatly in a fascinating blog last March, especially with the graphs from Tamara Davis but please keep talking about it! ("Cosmic Horizons" would make a great future talk to MAS.)

    I am interested in that poor little light photon - emitted from the Big Bang itself - that spends much of the time getting further away from us; but it crosses the Hubble Sphere several times and pushes on, eventually reaching the Solar System at the third attempt, after 130 Gyrs.

    It's scary to think that, if it started off its journey as a visible light photon, it would probably be red-shifted so far into the radio spectrum by the time it finally reached the Solar System about the year 116,000,000,000 CE, it must become stretched so much that it might not even be detectable. Wouldn't the amplitude of its waveform be nearly flat?

    (Even scarier is the notion that, in its own time frame, it got here in no time at all.)




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    1. You can think of the photon travelling from (near) the Big Bang to be a Cosmic Microwave Background photon. These were extremely high energy, but have been redshifted into the microwave part of the spectrum (for those) arriving today, and will be even more redshifted by 130 Gyrs. I don't know off the top of my head what the redshift would be by then, but it may be so large that the photon is unobservable.

      But the red lines are photon paths, and you could plonk an emitter (such as a star) at any point along the path and then follow the photon from then to now. The redshift would then be the ratio of the size of the photon when it is received, and the size of the universe when it was emitted.

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  4. This is getting silly. Think of the Lemaitre "hesitation" universe where the universe has decelerated expansion and then comes almost to a halt before accelerated expansion takes over. This phase can be arbitrarily long (which, of course, requires fine-tuning). During this phase, the Hubble constant becomes arbitrarily small, hence the Hubble radius becomes arbitrarily large. Let the universe hesitate for a long time so that one can see objects at far (proper) distances. when accelerated expansion kicks in, the Hubble sphere gets smaller, but of course the objects do not disappear from view.

    This is 1920s cosmology. OK, not everyone understood everything at first, but everything was understood by the 1960s at the latest. Why papers which disagree with Geraint and me on these issues get published in otherwise reputable journals is one of the big unsolved questions in cosmology! :-)

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  5. >> Why papers which disagree with Geraint and me on these issues get published in otherwise reputable journals is one of the big unsolved questions in cosmology! :-)

    Absolutely!!

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