## A Spot of Hype

Posted in Astrohype, The Universe and Stuff with tags , , on May 19, 2017 by telescoper

A few weeks ago a paper came out in Monthly Notices of the Royal Astronomical Society (accompanied by a press release from the Royal Astronomical Society) about a possible explanation for the now-famous cold spot in the cosmic microwave background sky that I’ve blogged about on a number of occasions:

If the standard model of cosmology is correct then a spot as cold as this and as large as this is quite a rare event, occurring only about 1% of the time in sky patterns simulated using the model assumptions. One possible explanation of this ( which I’ve discussed before) is that this feature is generated not by density fluctuations in the primordial plasma (which are thought to cause the variation of temperature of the cosmic microwave background across the sky), but by something much more recent in the evolution of the Universe, namely a local large void in the matter distribution which would cause a temperature fluctuation by the Sachs-Wolfe Effect.

The latest paper by Mackenzie et al. (which can be found on the arXiv here) pours enough cold water on that explanation to drown it completely and wash away the corpse. A detailed survey of the galaxy distribution in the direction of the cold spot shows no evidence for an under-density deep enough to affect the CMB. But if the cold spot is not caused by a supervoid, what is it caused by?

Right at the end of the paper the authors discuss a few alternatives,  some of them invoking exotic’ physics early in the Universe’s history. One such possibility arises if we live in an inflationary Universe in which our observable universe is just one of a (perhaps infinite) collection of bubble-like domains which are now causally disconnected. If our bubble collided with another bubble early on then it might distort the cosmic microwave background in our bubble, in much the same way that a collision with another car might damage your car’s bodywork.

For the record I’ve always found this explanation completely implausible. A simple energy argument suggests that if such a collision were to occur between two inflationary bubbles, it is much more likely to involve their mutual destruction than a small dint. In other words, both cars would be written off.

Nevertheless, the press have seized on this possible explanation, got hold of the wrong end of the stick and proceeded to beat about the bush with it. See, for example, the Independent headline: Mysterious ‘cold spot’ in space could be proof of a parallel universe, scientists say’.

No. Actually, scientists don’t say that. In particular, the authors of the paper don’t say it either. In fact they don’t mention proof’ at all. It’s pure hype by the journalists. I don’t blame Mackenzie et al, nor the RAS Press team. It’s just silly reporting.

Anyway, I’m sure I can hear you asking what I think is the origin of the cold spot. Well, the simple answer is that I don’t know for sure. The more complicated answer is that I strongly suspect that at least part of the explanation for why this patch of sky looks as cold as it does is tied up with another anomalous feature of the CMB, i.e. the hemispherical power asymmetry.

In the standard cosmological model the CMB fluctuations are statistically isotropic, which means the variance is the same everywhere on the sky. In observed maps of the microwave background, however, there is a slight but statistically significant variation of the variance, in such a way that the half of the sky that includes the cold spot has larger variance than the opposite half.

My suspicion is that the hemispherical power asymmetry is either an instrumental artifact (i.e. a systematic of the measurement) or is generated by improper substraction of foreground signals (from our galaxy or even from within the Solar system). Whatever causes it, this effect could well modulate the CMB temperature in such a way that it makes the cold spot look more impressive than it actually is. It seems to me that the cold spot could be perfectly consistent with the standard model if this hemispherical anomaly is taken into account. This may not be exotic’ or `exciting’ or feed the current fetish for the multiverse, but I think it’s the simplest and most probable explanation.

Call me old-fashioned.

## A Quite Interesting Question: How Loud Was the Big Bang?

Posted in The Universe and Stuff with tags , , , , , , , on March 16, 2017 by telescoper

I just found out this morning that this blog got a mention on the QI Podcast. It’s taken a while for this news to reach me, as the item concerned is two years old! You can find this discussion here, about 16 minutes in. And no, it’s not in connection with yawning psychopaths. It was about the vexed question of how loud was the Big Bang?

I’ve posted on this before (here and here)but since I’m very busy again today I  should recycle the discussion, and update it as it relates to the cosmic microwave background, which is what one of the things I work on on the rare occasions on which I get to do anything interesting.

As you probably know the Big Bang theory involves the assumption that the entire Universe – not only the matter and energy but also space-time itself – had its origins in a single event a finite time in the past and it has been expanding ever since. The earliest mathematical models of what we now call the  Big Bang were derived independently by Alexander Friedman and George Lemaître in the 1920s. The term “Big Bang” was later coined by Fred Hoyle as a derogatory description of an idea he couldn’t stomach, but the phrase caught on. Strictly speaking, though, the Big Bang was a misnomer.

Friedman and Lemaître had made mathematical models of universes that obeyed the Cosmological Principle, i.e. in which the matter was distributed in a completely uniform manner throughout space. Sound consists of oscillating fluctuations in the pressure and density of the medium through which it travels. These are longitudinal “acoustic” waves that involve successive compressions and rarefactions of matter, in other words departures from the purely homogeneous state required by the Cosmological Principle. The Friedman-Lemaitre models contained no sound waves so they did not really describe a Big Bang at all, let alone how loud it was.

However, as I have blogged about before, newer versions of the Big Bang theory do contain a mechanism for generating sound waves in the early Universe and, even more importantly, these waves have now been detected and their properties measured.

The above image shows the variations in temperature of the cosmic microwave background as charted by the Planck Satellite. The average temperature of the sky is about 2.73 K but there are variations across the sky that have an rms value of about 0.08 milliKelvin. This corresponds to a fractional variation of a few parts in a hundred thousand relative to the mean temperature. It doesn’t sound like much, but this is evidence for the existence of primordial acoustic waves and therefore of a Big Bang with a genuine “Bang” to it.

A full description of what causes these temperature fluctuations would be very complicated but, roughly speaking, the variation in temperature you corresponds directly to variations in density and pressure arising from sound waves.

So how loud was it?

The waves we are dealing with have wavelengths up to about 200,000 light years and the human ear can only actually hear sound waves with wavelengths up to about 17 metres. In any case the Universe was far too hot and dense for there to have been anyone around listening to the cacophony at the time. In some sense, therefore, it wouldn’t have been loud at all because our ears can’t have heard anything.

Setting aside these rather pedantic objections – I’m never one to allow dull realism to get in the way of a good story- we can get a reasonable value for the loudness in terms of the familiar language of decibels. This defines the level of sound (L) logarithmically in terms of the rms pressure level of the sound wave Prms relative to some reference pressure level Pref

L=20 log10[Prms/Pref].

(the 20 appears because of the fact that the energy carried goes as the square of the amplitude of the wave; in terms of energy there would be a factor 10).

There is no absolute scale for loudness because this expression involves the specification of the reference pressure. We have to set this level by analogy with everyday experience. For sound waves in air this is taken to be about 20 microPascals, or about 2×10-10 times the ambient atmospheric air pressure which is about 100,000 Pa.  This reference is chosen because the limit of audibility for most people corresponds to pressure variations of this order and these consequently have L=0 dB. It seems reasonable to set the reference pressure of the early Universe to be about the same fraction of the ambient pressure then, i.e.

Pref~2×10-10 Pamb.

The physics of how primordial variations in pressure translate into observed fluctuations in the CMB temperature is quite complicated, because the primordial universe consists of a plasma rather than air. Moreover, the actual sound of the Big Bang contains a mixture of wavelengths with slightly different amplitudes. In fact here is the spectrum, showing a distinctive signature that looks, at least in this representation, like a fundamental tone and a series of harmonics…

If you take into account all this structure it all gets a bit messy, but it’s quite easy to get a rough but reasonable estimate by ignoring all these complications. We simply take the rms pressure variation to be the same fraction of ambient pressure as the averaged temperature variation are compared to the average CMB temperature,  i.e.

Prms~ a few ×10-5Pamb.

If we do this, scaling both pressures in logarithm in the equation in proportion to the ambient pressure, the ambient pressure cancels out in the ratio, which turns out to be a few times 10-5. With our definition of the decibel level we find that waves of this amplitude, i.e. corresponding to variations of one part in a hundred thousand of the reference level, give roughly L=100dB while part in ten thousand gives about L=120dB. The sound of the Big Bang therefore peaks at levels just a bit less than 120 dB.

As you can see in the Figure above, this is close to the threshold of pain,  but it’s perhaps not as loud as you might have guessed in response to the initial question. Modern popular beat combos often play their dreadful rock music much louder than the Big Bang….

A useful yardstick is the amplitude  at which the fluctuations in pressure are comparable to the mean pressure. This would give a factor of about 1010 in the logarithm and is pretty much the limit that sound waves can propagate without distortion. These would have L≈190 dB. It is estimated that the 1883 Krakatoa eruption produced a sound level of about 180 dB at a range of 100 miles. The QI podcast also mentions  that blue whales make a noise that corresponds to about 188 decibels. By comparison the Big Bang was little more than a whimper..

PS. If you would like to read more about the actual sound of the Big Bang, have a look at John Cramer’s webpages. You can also download simulations of the actual sound. If you listen to them you will hear that it’s more of  a “Roar” than a “Bang” because the sound waves don’t actually originate at a single well-defined event but are excited incoherently all over the Universe.

## Fake News of the Holographic Universe

Posted in Astrohype, The Universe and Stuff with tags , , , , , , on February 1, 2017 by telescoper

It has been a very busy day today but I thought I’d grab a few minutes to rant about something inspired by a cosmological topic but that I’m afraid is symptomatic of malaise that extends far wider than fundamental science.

The other day I found a news item with the title Study reveals substantial evidence of holographic universe. You can find a fairly detailed discussion of the holographic principle here, but the name is fairly self-explanatory: the familiar hologram is a two-dimensional object that contains enough information to reconstruct a three-dimensional object. The holographic principle extends this to the idea that information pertaining to a higher-dimensional space may reside on a lower-dimensional boundary of that space. It’s an idea which has gained some traction in the context of the black hole information paradox, for example.

There are people far more knowledgeable about the holographic principle than me, but naturally what grabbed my attention was the title of the news item: Study reveals substantial evidence of holographic universe. That got me really excited, as I wasn’t previously aware that there was any observed property of the Universe that showed any unambiguous evidence for the holographic interpretation or indeed that models based on this model could describe the available data better than the standard ΛCDM cosmological model. Naturally I went to the original paper on the arXiv by Niayesh Ashfordi et al. to which the news item relates. Here is the abstract:

We test a class of holographic models for the very early universe against cosmological observations and find that they are competitive to the standard ΛCDM model of cosmology. These models are based on three dimensional perturbative super-renormalizable Quantum Field Theory (QFT), and while they predict a different power spectrum from the standard power-law used in ΛCDM, they still provide an excellent fit to data (within their regime of validity). By comparing the Bayesian evidence for the models, we find that ΛCDM does a better job globally, while the holographic models provide a (marginally) better fit to data without very low multipoles (i.e. l≲30), where the dual QFT becomes non-perturbative. Observations can be used to exclude some QFT models, while we also find models satisfying all phenomenological constraints: the data rules out the dual theory being Yang-Mills theory coupled to fermions only, but allows for Yang-Mills theory coupled to non-minimal scalars with quartic interactions. Lattice simulations of 3d QFT’s can provide non-perturbative predictions for large-angle statistics of the cosmic microwave background, and potentially explain its apparent anomalies.

The third sentence (highlighted) states explicitly that according to the Bayesian evidence (see here for a review of this) the holographic models do not fit the data even as well as the standard model (unless some of the CMB measurements are excluded, and then they’re only slightly better)

I think the holographic principle is a very interesting idea and it may indeed at some point prove to provide a deeper understanding of our universe than our current models. Nevertheless it seems clear to me that the title of this news article is extremely misleading. Current observations do not really provide any evidence in favour of the holographic models, and certainly not “substantial evidence”.

The wider point should be obvious. We scientists rightly bemoan the era of “fake news”. We like to think that we occupy the high ground, by rigorously weighing up the evidence, drawing conclusions as objectively as possible, and reporting our findings with a balanced view of the uncertainties and caveats. That’s what we should be doing. Unless we do that we’re not communicating science but engaged in propaganda, and that’s a very dangerous game to play as it endangers the already fragile trust the public place in science.

The authors of the paper are not entirely to blame as they did not write the piece that kicked off this rant, which seems to have been produced by the press office at the University of Southampton, but they should not have consented to it being released with such a misleading title.

## A Cosmic Microwave Background Dipole Puzzle

Posted in Cute Problems, The Universe and Stuff with tags , , , , , on October 31, 2016 by telescoper

The following is tangentially related to a discussion I had during a PhD examination last week, and I thought it might be worth sharing here to stimulate some thought among people interested in cosmology.

First here’s a picture of the temperature fluctuations in the cosmic microwave background from Planck (just because it’s so pretty).

The analysis of these fluctuations yields a huge amount of information about the universe, including its matter content and spatial geometry as well as the form of primordial fluctuations that gave rise to galaxies and large-scale structure. The variations in temperature that you see in this image are small – about one-part in a hundred thousand – and they show that the universe appears to be close to isotropic (at least around us).

I’ll blog later on (assuming I find time) on the latest constraints on this subject, but for the moment I’ll just point out something that has to be removed from the above map to make it look isotropic, and that is the Cosmic Microwave Background Dipole. Here is a picture (which I got from here):

This signal – called a dipole because it corresponds to a simple 180 degree variation across the sky – is about a hundred times larger than the “intrinsic” fluctuations which occur on smaller angular scales and are seen in the first map. According to the standard cosmological framework this dipole is caused by our peculiar motion through the frame in which microwave background photons are distributed homogeneously and isotropically. Had we no peculiar motion then we would be “at rest” with respect to this CMB reference frame so there would be no such dipole. In the standard cosmological framework this “peculiar motion” of ours is generated by the gravitational effect of local structures and is thus a manifestation of the fact that our universe is not homogeneous on small scales; by “small” I mean on the scales of a hundred Megaparsecs or so. Anyway, if you’re interested in goings-on in the very early universe or its properties on extremely large scales the dipole is thus of no interest and, being so large, it is quite easy to subtract. That’s why it isn’t there in maps such as the Planck map shown above. If it had been left in it would swamp the other variations.

Anyway, the interpretation of the CMB dipole in terms of our peculiar motion through the CMB frame leads to a simple connection between the pattern shown in the second figure and the velocity of the observational frame: it’s a Doppler Effect. We are moving towards the upper right of the figure (in which direction photons are blueshifted, so the CMB looks a bit hotter in that direction) and away from the bottom left (whence the CMB photons are redshifted so the CMB appears a bit cooler). The amplitude of the dipole implies that the Solar System is moving with a velocity of around 370 km/s with respect to the CMB frame.

Now 370 km/s is quite fast, but it’s much smaller than the speed of light – it’s only about 0.12%, in fact – which means that one can treat this is basically a non-relativistic Doppler Effect. That means that it’s all quite straightforward to understand with elementary physics. In the limit that v/c<<1 the Doppler Effect only produces a dipole pattern of the type we see in the Figure above, and the amplitude of the dipole is ΔT/T~v/c because all terms of higher order in v/c are negligibly smallFurthermore in this case the dipole is simply superimposed on the primordial fluctuations but otherwise does not affect them.

My question to the reader, i.e. you,  is the following. Suppose we weren’t travelling at a sedate 370 km/s through the CMB frame but instead enter the world of science fiction and take a trip on a spacecraft that can travel close to the speed of light. What would this do to the CMB? Would we still just see a dipole, or would we see additional (relativistic) effects? If there are other effects, what would they do to the pattern of “intrinsic” fluctuations?

## Cosmology: Galileo to Gravitational Waves – with Hiranya Peiris

Posted in The Universe and Stuff with tags , , , on September 9, 2016 by telescoper

Here’s another thing I was planning to post earlier in the summer, but for some reason forgot. It’s a video of a talk given at the Royal Institution earlier this year by eminent cosmologist Prof. Hiranya Peiris of University College London. The introduction to the talk goes like this:

Modern fundamental physics contains ideas just as revolutionary as those of Copernicus or Newton; ideas that may radically change our understanding of the world; ideas such as extra dimensions of space, or the possible existence of other universes.

Testing these concepts requires enormous energies, far higher than what is achievable by the Large Hadron Collider at CERN, and in fact, beyond any conceivable Earth-bound experiments. However, at the Big Bang, the Universe itself performed the ultimate experiment and left clues and evidence about what was behind the origin of the cosmos as we know it, and how it is evolving. And the biggest clue is the afterglow of the Big Bang itself.

In the past decade we have been able to answer age-old questions accurately, such as how old the Universe is, what it contains, and its destiny. Along with these answers have also come many exciting new questions. Join Hiranya Peiris to unravel the detective story, explaining what we have uncovered, and how we know what we know.

Hiranya Peiris is Professor of Astrophysics in the Astrophysics Group in the Department of Physics and Astronomy at University College London. She is also the Principal Investigator of the CosmicDawn project, funded by the European Research Council

She is also a member of the Planck Collaboration and of the ongoing Dark Energy Survey, the Dark Energy Spectroscopic Instrument and the Large Synoptic Survey Telescope. Her work both delves into the Cosmic Microwave Background and contributes towards the next generation galaxy surveys that will yield deep insights into the evolution of the Universe.

I’ve heard a lot of people talk about “Cosmic Dawn” but I’ve never met her…

Anyway, here is the video. It’s quite long (almost an hour) but very interesting and well-presented for experts and non-experts alike!

Update: I’ve just heard the news that Hiranya is shortly to take up a new job in Sweden as Director of the Oscar Klein Centre for Cosmoparticle Physics. Hearty congratulations and good luck to her!

## Blog Paper

Posted in Biographical, The Universe and Stuff with tags , , on April 12, 2016 by telescoper

I don’t often blog about my own research. To be honest that’s partly because I don’t get much time to do any. Fortunately, however, I have an excellent postdoctoral research assistant (Dipak) and some excellent collaborators. Anyway, I just heard yesterday that the following paper has been accepted for publication in the Journal of Cosmology and Astroparticle Physics (JCAP):

It’s not exactly a light read – it’s 32 pages long – but at least it gives the non-cosmology readers of this blog an idea of my research interests. Hopefully it won’t be too long before we can apply techniques such as those described in the above paper to real data!

Hopefully also in future I’ll be able to persuade my co-authors to submit to the Open Journal of Astrophysics!

## BICEP3 Cometh…

Posted in The Universe and Stuff with tags , , , , , on January 6, 2016 by telescoper

Back in the office after the Christmas and New Year break, with a mountain of stuff to work through..

Anyway, I saw this paper on the arXiv yesterday and thoought I’d share it here. It’s from a paper by Wu et al. entitled Initial Performance of BICEP3: A Degree Angular Scale 95 GHz Band Polarimeter.  The abstract follows:

BICEP3 is a 550 mm aperture telescope with cold, on-axis, refractive optics designed to observe at the 95 GHz band from the South Pole. It is the newest member of the BICEP/Keck family of inflationary probes specifically designed to measure the polarization of the cosmic microwave background (CMB) at degree-angular scales. BICEP3 is designed to house 1280 dual-polarization pixels, which, when fully-populated, totals to 9× the number of pixels in a single Keck 95 GHz receiver, thus further advancing the BICEP/Keck program’s 95 GHz mapping speed. BICEP3 was deployed during the austral summer of 2014-2015 with 9 detector tiles, to be increased to its full capacity of 20 in the second season. After instrument characterization measurements were taken, CMB observation commenced in April 2015. Together with multi-frequency observation data from Planck, BICEP2, and the Keck Array, BICEP3 is projected to set upper limits on the tensor-to-scalar ratio to r 0.03 at 95% C.L..

It all looks very promising, with science results likely to appear later this year, but who will win the race to find those elusive primordial B-modes?