Baby Universes in the Laboratory

This was on the new books table at our local bookstore. I had almost forgotten about doing an interview and corresponding with the author some time ago. See also here and here.

The book is a well-written overview of some of the more theoretical aspects of inflationary cosmology, the big bang, the multiverse, etc. It also fleshes out some of the individual stories of the physicists involved in this research.

Kirkus Reviews: ... In her elegant and perceptive book, Merali ... unpacks the science behind what we know about our universe’s beginnings and traces the paths that many renowned researchers have taken to translate these insights to new heights: the creation of a brand-new “baby” universe, and not an empty one, either, but one with its own physics, matter, and (possibly) life. ... Among the most significant scientific advances in the last half-century is the discovery that our universe is inflating exponentially, a theory that led to many more breakthroughs in physics and cosmology. Yet the big question—how did the universe form, triggering inflation to begin with?—remains opaque. Merali, who works at the Foundational Questions Institute, which explores the boundaries of physics and cosmology, effortlessly explains the complex theories that form the bedrock of this concept, and she brings to life the investigators who have dedicated much of their careers in pursuit of fundamental truths. She also neatly incorporates discussions of philosophy and religion—after all, nothing less than grand design itself is at stake here—without any heavy-handedness or agenda. Over the course of several years, she traveled the world to interview firsthand the most important figures behind the idea of laboratory universe creation ... and the anecdotes she includes surrounding these conversations make her portrait even more compelling.

Here are two illustrations of how a baby universe pinches off from the universe in which it was created. This is all calculable within general relativity, modulo an issue with quantum smoothing of a singularity. The remnant of the baby universe appears to outside observers as a black hole. But inside one finds an exponentially growing region of spacetime.

Seminars, Colloquia, and Slides I have known

I think I've made this Google drive folder publicly readable. It contains slides for many talks I've given over the years, going back almost to 2000 or so.

Topics include black hole information, monsters in curved space, entanglement entropy, dark energy, insider's guide to startups, the financial crisis of 2008, foundations of quantum mechanics, and more.

(Second slide is from this talk given at the Institute for Quantum Information at Caltech.)

Astrophysical Constraints on Dark Energy v2

This is v2 of a draft we posted earlier in the year. The new version has much more detail on whether rotation curve measurements of an isolated dwarf galaxy might be able to constrain the local dark energy density. As we state in the paper (c is the local dark energy density):

In Table V, we simulate the results of measurements on v 2 (r) with corresponding error of 1%. We take ρ0 ∼ 0.2 GeV cm−3 and Rs ∼ 0.795 kpc for the dwarf galaxies. We vary the number of satellites N and their (randomly generated) orbital radii. For example, at 95% confidence level, one could bound c to be positive using 5 satellites at r ∼ 1 − 10 kpc. In order to bound c close to its cosmological value, one would need, e.g., at least 5 satellites at r ∼ 10 − 20 kpc or 10 satellites at r ∼ 5 − 15 kpc. 

... In Table VI, we simulate the results from measurements on v2(r), assuming that the corresponding error is 5%. Again, we take ρ0 ∼ 0.2 GeV cm−3 and Rs ∼ 0.795 kpc for the dwarf galaxies. The table indicates that even at the sensitivity of 5%, one could rule out (at 95% confidence level) any Λ that is significantly larger than 1.58×10−84 GeV2 by using, e.g., 5 satellites at r ∼ 1−10 kpc. The very existence of satellites of dwarf galaxies (even those close to the Milky Way, and hence subject to significant tidal forces that limit r) provides an upper limit on the local dark energy density, probably no more than an order of magnitude larger than the cosmological value.  

Since we are not real astronomers, it is unclear to us whether measurements of the type described above are pure science fiction or something possible, say, in the next 10-20 years. Multiple conversations with astronomers (and referees) have failed to completely resolve this issue. Note that papers in reference [11] (Swaters et al.) report velocity measurements for satellites of dwarf galaxies at radii ~ 10 kpc with existing technology.

Astrophysical Constraints on Dark Energy

Chiu Man Ho, Stephen D. H. Hsu
(Submitted on 23 Jan 2015 (v1), last revised 3 Jul 2015 (this version, v2))

Dark energy (i.e., a cosmological constant) leads, in the Newtonian approximation, to a repulsive force which grows linearly with distance and which can have astrophysical consequences. For example, the dark energy force overcomes the gravitational attraction from an isolated object (e.g., dwarf galaxy) of mass 107M⊙ at a distance of 23 kpc. Observable velocities of bound satellites (rotation curves) could be significantly affected, and therefore used to measure or constrain the dark energy density. Here, {\it isolated} means that the gravitational effect of large nearby galaxies (specifically, of their dark matter halos) is negligible; examples of isolated dwarf galaxies include Antlia or DDO 190.

Astrophysical Repulsion from Dark Energy

The manifestation of dark energy on cosmological scales is well known: gravitational repulsion which leads to the accelerating expansion of the universe. Perhaps surprisingly, there are potentially observable effects on galactic length scales as well.

The Dark Force: Astrophysical Repulsion from Dark Energy (http://arxiv.org/abs/1501.05952)

Chiu Man Ho, Stephen D. H. Hsu

Dark energy (i.e., a cosmological constant) leads, in the Newtonian approximation, to a repulsive force which grows linearly with distance. We discuss possible astrophysical effects of this "dark" force. For example, the dark force overcomes the gravitational attraction from an object (e.g., dwarf galaxy) of mass $10^7 M_\odot$ at a distance of $~ 23$ kpc. It seems possible that observable velocities of bound satellites (rotation curves) could be significantly affected, and therefore used to measure the dark energy density.

Instability of Quantum de Sitter Spacetime

New paper! We show that quantum effects (in particular, the horizon temperature originally discovered by Gibbons and Hawking) modify the geometry of de Sitter spacetime.

Instability of Quantum de Sitter Spacetime (http://arxiv.org/abs/1501.00708)

Chiu Man Ho, Stephen D. H. Hsu

Quantized fields (e.g., the graviton itself) in de Sitter (dS) spacetime lead to particle production: specifically, we consider a thermal spectrum resulting from the dS (horizon) temperature. The energy required to excite these particles reduces slightly the rate of expansion and eventually modifies the semiclassical spacetime geometry. The resulting manifold no longer has constant curvature nor time reversal invariance, and back-reaction renders the classical dS background unstable to perturbations. In the case of AdS, there exists a global static vacuum state; in this state there is no particle production and the analogous instability does not arise.

Implications of cosmological tensor modes

New paper! What can we conclude about high energy physics from the BICEP2 observations of a cosmological tensor mode background? See earlier post Patterns on the Sky.

Does the BICEP2 Observation of Cosmological Tensor Modes Imply an Era of Nearly Planckian Energy Densities?  (arXiv:1404.0745)

Chiu Man Ho, Stephen D. H. Hsu

BICEP2 observations, interpreted most simply, suggest an era of inflation with energy densities of order ($10^{16}\, {\rm GeV})^4$, not far below the Planck density. However, models of TeV gravity with large extra dimensions might allow a very different interpretation involving much more modest energy scales. We discuss the viability of inflation in such models, and conclude that existing scenarios do not provide attractive alternatives to single field inflation in four dimensions. Because the detection of tensor modes strengthens our confidence that inflation occurred, it disfavors models of large extra dimensions, at least for the moment.

Patterns on the sky

I'm busy reviewing ~200 promotion and tenure cases for my day job, so I don't have much time to post about the BICEP2 observation of primordial gravitational waves via their effect on the polarization of the cosmic microwave background (CMB).

Instead, I refer you to Sean Carroll, Lubos Motl and Liam McAllister (guest poster at Lubos' blog).

Assuming the result holds up, it strongly supports inflationary cosmology, and indicates that the inflation scale is only about 2 orders of magnitude below the Planck scale ~ 10^19 GeV (which would, presumably, turn out to be the true scale of quantum gravity).

In inflationary cosmology the gravitational waves which left the polarization signal arise from quantum fluctuations in de Sitter space. As with the CMB temperature, observers on different branches of the wavefunction of the universe see distinct polarization patterns on the sky. Since the CMB temperature fluctuations track energy density, these different observers also see distinct patterns of galaxy formation. In fact, whether or not an observer (a planet or galaxy) exists in a particular region of spacetime depends on the branch of the wavefunction (i.e., on a measurement outcome). I can't tell a Copenhagen story that makes sense of this -- there is no way to place observers like ourselves outside of the quantum state describing the CMB!

I guess I've said this all before 8-)

In fact, the interpretation of quantum mechanics is not entirely disconnected from practical issues in cosmology. The cosmic microwave background data favors inflationary cosmology, in which the density perturbations in the early universe originate in the quantum fluctuations of the inflaton field itself. It is very hard to fit this into the Copenhagen view -- what "collapses" the wavefunction of the inflaton? There are no "observers" in the early universe, and the locations of "observers" (such as humans) are determined by the density perturbations themselves: galaxies, stars and planets are found in the overdense regions, but quantum mechanics itself decides which regions are overdense; there is no classical system "outside" the universe! It seems much more natural to note that differential scattering of gravitons due to more or less energy density in a particular region separates the inflaton wavefunction into decoherent branches. (The gravitons decohere the inflaton state vector through interactions.) But this is accomplished through unitary evolution and does not require von Neumann projection or "collapse". Other observers, living on other branches of the wavefunction, see a different CMB pattern on the sky.

Look up, not down!

Our biggest astrophysics projects are a rounding error in the spook budget.

NYTimes: The phone call came like a bolt out of the blue, so to speak, in January 2011. On the other end of the line was someone from the National Reconnaissance Office, which operates the nation’s fleet of spy satellites. They had some spare, unused “hardware” to get rid of. Was NASA interested? 

... Sitting in a clean room in upstate New York were a pair of telescopes the same size as the famed Hubble Space Telescope, but which had been built to point down at the Earth, instead of up at the heavens. 

... to turn one of the telescopes loose on the cosmos, pointing in its rightful direction, outward, to investigate the mysterious dark energy that is speeding up the expansion of the universe. If the plan succeeds — and Congress, the Office of Management and Budget and the Academy of Sciences have yet to sign on — it could shave hundreds of millions of dollars and several years off a quest that many scientists say is the most fundamental of our time and that NASA had said it could not undertake until 2024 at the earliest.

Entanglement and Decoherence

My preprint http://arxiv.org/pdf/1205.1584v1.pdf (which appeared yesterday evening on arxiv) has already elicited a response from the brilliant and eccentric Lubos Motl. Lubos believes in the "subjective" interpretation of the quantum state, so objects to the idea of a unitarily-evolving wavefunction describing all degrees of freedom in the universe. (See here for more discussion.) I, on the other hand, am willing to consider the possibility that many worlds is correct. Here is how Lubos characterizes the disagreement:

Buniy and Hsu also seem to be confused about the topics that have been covered hundreds of times on this blog. In particular, the right interpretation of the state is a subjective one. Consequently, all the properties of a state – e.g. its being entangled – are subjective as well. They depend on what the observer just knows at a given moment. Once he knows the detailed state of objects or observables, their previous entanglement becomes irrelevant. 

... When I read papers such as one by Buniy and Hsu, I constantly see the wrong assumption written everything in between the lines – and sometimes inside the lines – that the wave function is an objective wave and one may objectively discuss its properties. Moreover, they really deny that the state vector should be updated when an observable is changed. But that's exactly what you should do. The state vector is a collection of complex numbers that describe the probabilistic knowledge about a physical system available to an observer and when the observer measures an observable, the state instantly changes because the state is his knowledge and the knowledge changes!

In the section of our paper on Schmidt decomposition, we write

A measurement of subsystem A which determines it to be in state ψ^(n)_A implies that the rest of the universe must be in state ψ^(n)_B. For example, A might consist of a few spins [9]; it is interesting, and perhaps unexpected, that a measurement of these spins places the rest of the universe into a particular state ψ^(n)_B. As we will see below, in the cosmological context these modes are spread throughout the universe, mostly beyond our horizon. Because we do not have access to these modes, they do not necessarily prevent us from detecting A in a superposition of two or more of the ψ^(n)_A. However, if we had sufficient access to B degrees of freedom (for example, if the relevant information differentiating between ψ^(n)_A states is readily accessible in our local environment or in our memory records), then the A system would decohere into one of the ψ^(n)_A.

This discussion makes it clear that ψ describes all possible branches of the wavefunction, including those that may have already decohered from each other: it describes not just the subjective experience of one observer, but of all possible observers. If we insist on removing decohered branches from the wavefunction (e.g., via collapse or von Neumann projection), then much of the entanglement we discuss in the paper is also excised. However, if we only remove branches that are inconsistent with the observations of a specific single observer, most of it will remain. Note decoherence is a continuous and (in principle) reversible phenomenon, so (at least within a unitary framework) there is no point at which one can say two outcomes have entirely decohered -- one can merely cite the smallness of overlap between the two branches or the level of improbability of interference between them.

I don't think Lubos disagrees with the mathematical statements we make about the entanglement properties of ψ. He may claim that these entanglement properties are not subject to experimental test. At least in principle, one can test whether systems A and B, which are in two different horizon volumes at cosmological time t1, are entangled. We have to wait until some later time t2, when there has been enough time for classical communication between A and B, but otherwise the protocol for determining entanglement is the usual one.

If we leave aside cosmology and consider, for example, the atoms or photons in a box, the same formalism we employ shows that there is likely to be widespread entanglement among the particles. In principle, an experimentalist who is outside the box can test whether the state ψ describing the box is "typical" (i.e., highly entangled) by making very precise measurements.

See stackexchange for more discussion.

Everything is Entangled

This paper will be available tomorrow at the link.

http://arxiv.org/abs/1205.1584 

Everything is Entangled 

Roman V. Buniy, Stephen D.H. Hsu 

We show that big bang cosmology implies a high degree of entanglement of particles in the universe. In fact, a typical particle is entangled with many particles far outside our horizon. However, the entanglement is spread nearly uniformly so that two randomly chosen particles are unlikely to be directly entangled with each other -- the reduced density matrix describing any pair is likely to be separable.

From the introduction:

Ergodicity and properties of typical pure states 

When two particles interact, their quantum states generally become entangled. Further interaction with other particles spreads the entanglement far and wide. Subsequent local manipulations of separated particles cannot, in the absence of quantum communication, undo the entanglement. We know from big bang cosmology that our universe was in thermal equilibrium at early times, and we believe, due to the uniformity of the cosmic microwave background, that regions which today are out of causal contact were once in equilibrium with each other. Below we show that these simple observations allow us to characterize many aspects of cosmological entanglement. 

We will utilize the properties of typical pure states in quantum mechanics. These are states which dominate the Hilbert measure. The ergodic theorem proved by von Neumann implies that under Schrodinger evolution most systems spend almost all their time in typical states. Indeed, systems in thermal equilibrium have nearly maximal entropy and hence must be typical. Typical states are maximally entangled (see below) and the approach to equilibrium can be thought of in terms of the spread of entanglement. ...

Professor Buniy in action! (Working on this research.)

Nobel Prizes 2011

I was a bit busy last week, with a visitor, posting a paper, etc. so I didn't get to comment on the Nobel prizes.

The dark energy prize is richly deserved (see slides from a colloquium on dark energy I've given a few times; includes above figure). These guys have discovered where most of the energy in the universe is, and may have determined the ultimate fate of the universe on cosmological scales. I note Saul Perlmutter was awarded 1/2 the prize and the other two guys each received 1/4. This may seem like petty credit splitting, but in this case it is appropriate as Perlmutter's group at LBNL have been working on supernova astronomy for a long time trying to get it to work. (Since when I was a grad student!) Perlmutter attributes the original idea to Luis Alvarez, perhaps the greatest experimentalist of the 20th century.

In finding that the universe is on a path to runaway expansion, you had to find type Ia supernovae, which can act as distance markers. How did you get involved with supernova searching?

I was at the University of California at Berkeley for graduate school. One of the heroes here at Berkeley is Luis Alvarez. The tradition that he started is looking for interesting science no matter where it is and then finding tools to do those things. For example, he invented one of the first steady cams.

One of his protégés was my professor, Richard Muller. There was a project to do a superautomated supernova search that Luis Alvarez had suggested to Rich. They had just done one of the first adaptive-optics experiments.

...

To what do you most attribute your scientific success?

I think the biggest thing is, first of all, being willing to learn things, being willing to pick up a new area, but also just being able to work with other people. Most of these jobs are too big for any one person. You end up trying to find a team of people who are as excited as you are and want to push the technique forward. I'm always struck by the fact that the image of the scientist is as a lone person wearing a lab jacket in the lab by themselves for hours, whereas my sense is that maybe the single most important thing for a scientist, aside from being able to think of good questions, is figuring out good people to work with and enjoying the process of inventing ideas together with other people.

You can add one more Nobel prize to the Berkeley lab collection:




I don't have too much to say about the quasicrystal prize, except that there are several curious aspects (this is mostly second hand stuff I picked up from colleagues): 1. the chemists gave a prize for a physics discovery, and seem to have botched the job: 2. they left out the theorist who was instrumental in convincing people that Shechtman's result was for real (Steinhardt had worked out the theory of quasicrystals out already, and even coined the name!) and 3. Shechtman's group at NIST (where he made the discovery) didn't believe the result and his boss kicked him out!

Genomics in Taipei, dark energy in Shanghai

Next Tuesday I will give the following colloquium at National Taiwan University (Taida).

Investigating the genetic basis of intelligence

I begin with a brief review of psychometric results concerning intelligence (sometimes referred to as the g factor, or IQ), emphasizing the stability, validity (predictive power) and heritability of adult IQ. Next, I discuss ongoing Genome Wide Association Studies which investigate the genetic basis of intelligence. Due mainly to the rapidly decreasing cost of sequencing (currently below $5k per genome), it is likely that within the next 5-10 years we will identify loci which account for a significant fraction of total IQ variation. Finally, I discuss the near term possibility of genetic engineering for intelligence.

This talk is aimed at physicists and should be accessible without specialized background in psychology or biology.

On Friday I'll be talking about dark energy (slides) at this meeting at Shanghai Jiao Tong University.

Dark Energy talk

Slides for the talk I'm giving Saturday at this meeting. The talk is titled Dark Energy, with Signatures.

Dark energy, with Signatures

New paper! This was selected for Honorable Mention in the 2010 Gravity Research Foundation Essay awards.

Dark Energy, with Signatures

http://arxiv.org/abs/1005.3038

Sourish Dutta, Stephen D. H. Hsu, Robert J. Scherrer


We propose a class of simple dark energy models which predict a late-time dark radiation component and a distinctive time-dependent equation of state $w(z)$ for redshift $z < 3$. The dark energy field can be coupled strongly enough to Standard Model particles to be detected in colliders, and the model requires only modest additional particle content and little or no fine-tuning other than a new energy scale of order milli-electron volts.

What is the entropy of the universe?

New paper! You can find some additional discussion on Cosmic Variance.

What is the entropy of the universe?

Paul Frampton, Stephen D.H. Hsu, Thomas W. Kephart, David Reeb


Abstract: Standard calculations suggest that the entropy of the universe is dominated by black holes, although they comprise only a tiny fraction of its total energy. We give a physical interpretation of this result. Statistical entropy is the logarithm of the number of microstates consistent with the observed macroscopic properties of a system, hence a measure of uncertainty about its precise state. The largest uncertainty in the present and future state of the universe is due to the (unknown) internal microstates of its black holes. We also discuss the qualitative gap between the entropies of black holes and ordinary matter.