# Please shoot this, somebody (Laura Mersini-Houghton paper)

1. Sep 3, 2006

### marcus

I noticed this on arxiv today. It is by a legitimate academic researcher and it MAY be reasonable for all I know. I can't judge. She is at the UNC-Chapel Hill department of Physics and Astronomy. But my initial impression is weirdness. I would like to be able to go to sleep tonight confident that it is OK to ignore this paper. Anybody want to dispose of it?

http://arxiv.org/abs/gr-qc/0609006
The Arrow of Time Forbids a Positive Cosmological Constant $\Lambda$
Laura Mersini-Houghton
6 pages

"Motivated by the mounting evidence for dark energy, here we explore the consequences of a fundamental cosmological constant $\Lambda$ for our universe. We show that when the gravitational entropy of a pure DeSitter state ultimately wins over matter, then the thermodynamic arrow of time in our universe must reverse in scales of order a Hubble time. This phenomenon arises from the gravitational instabilities that develop during a DeSitter epoch and turn catastrophic. A reversed arrow of time is clearly in disagreement with observations. Thus we are led to conclude: Nature forbids a fundamental $\Lambda$. Or else general relativity must be modified in the IR regime when $\Lambda$ dominates the expansion of the Universe."

Last edited: Sep 3, 2006
2. Sep 4, 2006

### oldman

Before the article by Laura Mersini-Houghton is ripped to pieces by the defenders of conventional cosmological faith, probably on technical grounds that I won't understand, let me take issue with one point it makes, namely that:

How do we know the direction of the arrow of time?

If we ourselves are an integral part of an holistic universe, reversing the arrow of time will affect all we are, as well as the universe we perceive. Our memories of the past may be created by physical processes that we call "irreversible", but they can only be so described relative to the arrow of time itself. We can't tell what this direction is, in an absolute sense, by perceiving that the total entropy of the universe increases relative to our perception of the flow of time.

On the face of it, Laura Mersini-Houghton's argument is as unsound as the proposal of Philip Gosse, who in 1857 argued that the world (and the Universe?) was created only a few thousand years ago (say in 4004 B.C.); complete with all that was needed for it to function just as we find it does, as I mentioned in another thread.

Perhaps Laura Mersini-Houghton's article can be shown to be wrong for better reasons. Or perhaps not!

I'll follow this thread with interest, Marcus.

Last edited: Sep 4, 2006
3. Sep 4, 2006

### SpaceTiger

Staff Emeritus
Although I really do hope we measure a non-zero dw/dz (cosmological constants are so boring), I'm not entirely convinced of what this paper concludes. It seems to me that all of this analysis was done in DeSitter space...and we don't live in DeSitter space. If there is a cosmological constant, then DeSitter space will be approached as the matter energy density becomes negligibly small. If their analysis is correct, I wouldn't expect any "time reversals" until some time in the future...

This also concerned me, though if you're right, then the paper would seem to predict a contracting, rather than expanding, universe. This would also be in contradiction with observations.

4. Sep 4, 2006

### chronon

Just as a generation ago people began to worry about the entropy of black holes, now they seem to be beginning to worry about the entropy of the cosmological event horizon, which unlike the case of black holes will not evaporate away. That seems to be what this paper is about. See also
This thread and my article The Cosmological Event Horizon

Note:arxiv is not peer reviewed, but has an endorsement system. Established academics, such as the author of the paper in question, are generally able to endorse their own papers.

5. Sep 5, 2006

### oldman

Now that some Forum Folk have had their say about the refereeing process -- or lack of it --- how about getting back to the validity of the arguments in Laura Mersini-Houghton's article?

6. Sep 5, 2006

### SpaceTiger

Staff Emeritus
Last edited by a moderator: Apr 22, 2017