Archive for the Astrohype Category

Gas Filaments in the Cosmic Web

Posted in Astrohype, The Universe and Stuff with tags , , , , , on October 4, 2019 by telescoper

I saw that there’s a new paper that has just been published in the journal Science by Umehata et al with the title Gas filaments of the cosmic web located around active galaxies in a protocluster. In case you run into a paywall at Science, you may of course, find the paper on the arXiv here.

The abstract reads:

Cosmological simulations predict the Universe contains a network of intergalactic gas filaments, within which galaxies form and evolve. However, the faintness of any emission from these filaments has limited tests of this prediction. We report the detection of rest-frame ultraviolet Lyman-alpha radiation from multiple filaments extending more than one megaparsec between galaxies within the SSA 22 proto-cluster at a redshift of 3.1. Intense star formation and supermassive black-hole activity is occurring within the galaxies embedded in these structures, which are the likely sources of the elevated ionizing radiation powering the observed Lyman-alpha emission. Our observations map the gas in filamentary structures of the type thought to fuel the growth of galaxies and black holes in massive proto-clusters.

The existence of a complex cosmic web of filaments and voids has been known about for some time as it is revealed on large scales by the distribution of galaxies through redshift surveys:

You can see all my posts agged with `Cosmic Web’ here. There are also good theoretical reasons (besides numerical simulations) for believing this is what the large-scale distribution of matter should look like. Roughly speaking, dense knots of matter lie at the vertices of a three-dimensional pattern traced out by one-dimensional structures.

We have also known for some time, however, that there is more going on in cosmic structure than is revealed by light from stars in galaxies. In particular the way gas flows along the filaments into the knots plays an important role in galaxy and cluster formation. This paper reveals the distribution of gas around a giant cluster that has formed at such a node using observations made using the European Southern Observatory’s MUSE instrument.

Here’s a pretty picture:

I found out about this paper from a news piece in the Guardian with the title Scientists observe mysterious cosmic web directly for first time. That’s sufficiently misleading for me to cross-file the paper under `Astrohype’ because, as I explained above, we have been observing the cosmic web for decades. It is however only just becoming possible to observe the diffuse gas rather than having to join the dots between the galaxies so it is an exciting result. My complaint, I suppose, is that the word `directly’ is doing a lot of heavy lifting in the title!

Results from the Event Horizon Telescope

Posted in Astrohype, The Universe and Stuff with tags , , on April 10, 2019 by telescoper

Following yesterday’s little teaser, let me point out that there is a press conference taking place today (at 2pm Irish Summer Time, that’s 3pm Brussels) to announce a new result from the Event Horizon Telescope. The announcement will be streamed live here.

Sadly, I’m teaching at the time of the press conference so I won’t be able to watch, but that doesn’t mean that you shouldn’t!

I’ll post pictures and comments when I get back. Watch this space. Or you could watch this video..

UPDATE: Well, there we are. Here is the image of the `shadow’ of the event horizon around the black hole in M87:

The image is about 42 micro arcseconds across. I guess to people brought up on science fiction movies with fancy special effects the image is probably a little underwhelming, but it really is an excellent achievement to get that resolution. Above all, it’s a great example of scientific cooperation – 8 different telescopes all round the world. The sizeable European involvement received a substantial injection of funding from the European Union too!

Other parameters are here:

The accompanying EU press release is here. Further information can be found here. The six publications relating to this result can be found here:

BICEP2: Is the Signal Cosmological?

Posted in Astrohype, The Universe and Stuff with tags , , on March 28, 2019 by telescoper

An article in Physics Today just reminded me just now that I have missed the fifth anniversary of the BICEP2 announcement of `the detection of primordial gravitational waves’. I know I’m a week but I thought I’d reblog the post I wrote on March 19th 2014.You will see that I was sceptical…

..and it subsequently turned out that I was right to be so.

In the Dark

I have a short gap in my schedule today so I thought I would use it to post a short note about the BICEP2 results announced to great excitement on Monday.

There has been a great deal of coverage in the popular media about a “Spectacular Cosmic Discovery” and this is mirrored by excitement at a more technical level about the theoretical implications of the BICEP2 results. Having taken a bit of time out last night to go through the discovery paper, I think I should say that I think all this excitement is very premature. In that respect I agree with the result of my straw poll.

First of all let me make it clear that the BICEP2 experiment is absolutely superb. It was designed and built by top-class scientists and has clearly functioned brilliantly to improve its sensitivity so much that it has gone so…

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The Negative Mass Bug

Posted in Astrohype, Open Access, The Universe and Stuff with tags , , , , , on February 25, 2019 by telescoper

You may have noticed that some time ago I posted about  a paper by Jamie Farnes published in Astronomy & Astrophysics but available on the arXiv here which entails a suggestion that material with negative mass might account for dark energy and/or dark matter.

Here is the abstract of said paper:

Dark energy and dark matter constitute 95% of the observable Universe. Yet the physical nature of these two phenomena remains a mystery. Einstein suggested a long-forgotten solution: gravitationally repulsive negative masses, which drive cosmic expansion and cannot coalesce into light-emitting structures. However, contemporary cosmological results are derived upon the reasonable assumption that the Universe only contains positive masses. By reconsidering this assumption, I have constructed a toy model which suggests that both dark phenomena can be unified into a single negative mass fluid. The model is a modified ΛCDM cosmology, and indicates that continuously-created negative masses can resemble the cosmological constant and can flatten the rotation curves of galaxies. The model leads to a cyclic universe with a time-variable Hubble parameter, potentially providing compatibility with the current tension that is emerging in cosmological measurements. In the first three-dimensional N-body simulations of negative mass matter in the scientific literature, this exotic material naturally forms haloes around galaxies that extend to several galactic radii. These haloes are not cuspy. The proposed cosmological model is therefore able to predict the observed distribution of dark matter in galaxies from first principles. The model makes several testable predictions and seems to have the potential to be consistent with observational evidence from distant supernovae, the cosmic microwave background, and galaxy clusters. These findings may imply that negative masses are a real and physical aspect of our Universe, or alternatively may imply the existence of a superseding theory that in some limit can be modelled by effective negative masses. Both cases lead to the surprising conclusion that the compelling puzzle of the dark Universe may have been due to a simple sign error.

Well there’s a new paper just out on the arXiv by Hector Socas-Navarro with the abstract

A recent work by Farnes (2018) proposed an alternative cosmological model in which both dark matter and dark energy are replaced with a single fluid of negative mass. This paper presents a critical review of that model. A number of problems and discrepancies with observations are identified. For instance, the predicted shape and density of galactic dark matter halos are incorrect. Also, halos would need to be less massive than the baryonic component or they would become gravitationally unstable. Perhaps the most challenging problem in this theory is the presence of a large-scale version of the `runaway’ effect, which would result in all galaxies moving in random directions at nearly the speed of light. Other more general issues regarding negative mass in general relativity are discussed, such as the possibility of time-travel paradoxes.

Among other things there is this:

After initially struggling to reproduce the F18 results, a careful inspection of his source code revealed a subtle bug in the computation of the gravitational acceleration. Unfortunately, the simulations in F18 are seriously compromised by this coding error whose effect is that the gravitational force decreases with the inverse of the distance, instead of the distance squared.

Oh dear.

I don’t think I need go any further into this particular case, which would just rub salt into the wounds of Farnes (2018) but I will make a general comment. Peer review is the best form of quality stamp that we have but, as this case demonstrates, it is by no means flawless. The paper by Farnes (2018) was refereed and published, but is now shown to be wrong*. Just as authors can make mistakes so can referees. I know I’ve screwed up as a referee in the past so I’m not claiming to be better than anyone in saying this.

*This claim is contested: see the comment below.

I don’t think the lesson is that we should just scrap peer review, but I do think we need to be more imaginative about how it is used than just relying on one or two individuals to do it. This case shows that science eventually works, as the error was found and corrected, but that was only possible because the code used by Farnes (2018) was made available for scrutiny. This is not always what happens. I take this as a vindication of open science, and an example of why scientists should share their code and data to enable others to check the results. I’d like to see a system in which papers are not regarded as `final’ documents but things which can be continuously modified in response to independent scrutiny, but that would require a major upheaval in academic practice and is unlikely to happen any time soon.

In this case, in the time since publication there has been a large amount of hype about the Farnes (2018) paper, and it’s unlikely that any of the media who carried stories about the results therein will ever publish retractions. This episode does therefore illustrate the potentially damaging effect on public trust that the excessive thirst for publicity can have. So how do we balance open science against the likelihood that wrong results will be taken up by the media before the errors are found? I wish I knew!

Negative Mass, Phlogiston and the State of Modern Cosmology

Posted in Astrohype, The Universe and Stuff with tags , , on December 7, 2018 by telescoper

A graphical representation of something or other.

I’ve noticed a modest amount of hype – much of it gibberish – going around about a paper published in Astronomy & Astrophysics but available on the arXiv here which entails a suggestion that material with negative mass might account for dark energy and/or dark matter. Here is the abstract of the paper:

Dark energy and dark matter constitute 95% of the observable Universe. Yet the physical nature of these two phenomena remains a mystery. Einstein suggested a long-forgotten solution: gravitationally repulsive negative masses, which drive cosmic expansion and cannot coalesce into light-emitting structures. However, contemporary cosmological results are derived upon the reasonable assumption that the Universe only contains positive masses. By reconsidering this assumption, I have constructed a toy model which suggests that both dark phenomena can be unified into a single negative mass fluid. The model is a modified ΛCDM cosmology, and indicates that continuously-created negative masses can resemble the cosmological constant and can flatten the rotation curves of galaxies. The model leads to a cyclic universe with a time-variable Hubble parameter, potentially providing compatibility with the current tension that is emerging in cosmological measurements. In the first three-dimensional N-body simulations of negative mass matter in the scientific literature, this exotic material naturally forms haloes around galaxies that extend to several galactic radii. These haloes are not cuspy. The proposed cosmological model is therefore able to predict the observed distribution of dark matter in galaxies from first principles. The model makes several testable predictions and seems to have the potential to be consistent with observational evidence from distant supernovae, the cosmic microwave background, and galaxy clusters. These findings may imply that negative masses are a real and physical aspect of our Universe, or alternatively may imply the existence of a superseding theory that in some limit can be modelled by effective negative masses. Both cases lead to the surprising conclusion that the compelling puzzle of the dark Universe may have been due to a simple sign error.

For a skeptical commentary on this work, see here.

The idea of negative mass is no by no means new, of course. If you had asked a seventeenth century scientist the question “what happens when something burns?”  the chances are the answer would  have involved the word phlogiston, a name derived from the Greek  φλογιστόν, meaning “burning up”. This “fiery principle” or “element” was supposed to be present in all combustible materials and the idea was that it was released into air whenever any such stuff was ignited. The act of burning separated the phlogiston from the dephlogisticated “true” form of the material, also known as calx.

The phlogiston theory held sway until  the late 18th Century, when Antoine Lavoisier demonstrated that combustion results in an increase in weight implying an increase in mass of the material being burned. This poses a serious problem if burning also involves the loss of phlogiston unless phlogiston has negative mass. However, many serious scientists of the 18th Century, such as Georg Ernst Stahl, had already suggested that phlogiston might have negative weight or, as he put it, `levity’. Nowadays we would probably say `anti-gravity.

Eventually, Joseph Priestley discovered what actually combines with materials during combustion:  oxygen. Instead of becoming dephlogisticated, things become oxidised by fixing oxygen from air, which is why their weight increases. It’s worth mentioning, though, the name that Priestley used for oxygen was in fact “dephlogisticated air” (because it was capable of combining more extensively with phlogiston than ordinary air). He  remained a phlogistonian longer after making the discovery that should have killed the theory.

The standard cosmological model involves the hypothesis that about 75% of the energy budget of the Universe is in the form of “dark energy”. We don’t know much about what this is, except that in order to make our current understanding work out it has to act like a source of anti-gravity. It does this by violating the strong energy condition of general relativity.

Dark energy is needed to reconcile three basic measurements: (i) the brightness distant supernovae that seem to indicate the Universe is accelerating (which is where the anti-gravity comes in); (ii) the cosmic microwave background that suggests the Universe has flat spatial sections; and (iii) the direct estimates of the mass associated with galaxy clusters that accounts for about 25% of the mass needed to close the Universe.

A universe without dark energy appears not to be able to account for these three observations simultaneously within our current understanding of gravity as obtained from Einstein’s theory of general relativity.

I’ve blogged before, with some levity of my own, about how uncomfortable this dark energy makes me feel. It makes me even more uncomfortable that such an enormous  industry has grown up around it and that its existence is accepted unquestioningly by so many modern cosmologists.

Isn’t there a chance that, with the benefit of hindsight, future generations will look back on dark energy in the same way that we now see the phlogiston theory?

Or maybe, as the paper that prompted this piece might be taken to suggest, the dark energy really is something like phlogiston. At least I prefer the name to quintessence. However, I think the author has missed a trick. I think to create a properly trendy cosmological theory he should include the concept of supersymmetry, according to which there should be a Fermionic counterpart of phlogiston called the phlogistino..

Hawking Points in the CMB Sky?

Posted in Astrohype, Bad Statistics, The Universe and Stuff with tags , on October 30, 2018 by telescoper

As I wait in Cardiff Airport for a flight back to civilization, I thought I’d briefly mention a paper that appeared on the arXiv this summer. The abstract of this paper (by Daniel An, Krzysztof A. Meissner and Roger Penrose) reads as follows:

This paper presents powerful observational evidence of anomalous individual points in the very early universe that appear to be sources of vast amounts of energy, revealed as specific signals found in the CMB sky. Though seemingly problematic for cosmic inflation, the existence of such anomalous points is an implication of conformal cyclic cosmology (CCC), as what could be the Hawking points of the theory, these being the effects of the final Hawking evaporation of supermassive black holes in the aeon prior to ours. Although of extremely low temperature at emission, in CCC this radiation is enormously concentrated by the conformal compression of the entire future of the black hole, resulting in a single point at the crossover into our current aeon, with the emission of vast numbers of particles, whose effects we appear to be seeing as the observed anomalous points. Remarkably, the B-mode location found by BICEP 2 is at one of these anomalous points.

The presence of Roger Penrose in the author list of this paper is no doubt a factor that contributed to the substantial amount of hype surrounding it, but although he is the originator of the Conformal Cyclic Cosmology I suspect he didn’t have anything to do with the data analysis presented in the paper as, great mathematician though he is, data analysis is not his forte.

I have to admit that I am very skeptical of the claims made in this paper – as I was in the previous case of claims of a evidence in favour of the Penrose model. In that case the analysis was flawed because it did not properly calculate the probability of the claimed anomalies in the standard model of cosmology. Moreover, the addition of a reference to BICEP2 at the end of the abstract doesn’t strengthen the case. The detection claimed by BICEP2 was (a) in polarization not in temperature and (b) is now known to be consistent with galactic foregrounds.

I will, however, hold my tongue on these claims, at least for the time being. I have an MSc student at Maynooth who is going to try to reproduce the analysis (which is not trivial, as the description in the paper is extremely vague). Watch this space.

EDGES and Foregrounds

Posted in Astrohype, The Universe and Stuff with tags , , , on September 3, 2018 by telescoper

Earlier this year I wrote a brief post about paper by Bowman et al. from the EDGES experiment that had just come out in Nature reportining the detection of a flattened absorption profile in the sky-averaged radio spectrum, centred at a frequency of 78 megahertz, largely consistent with expectations for the 21-centimetre signal induced by early stars. It caused a lot of excitement at the time; see, e.g., here.
The key plot from the paper is this:

At the time I said that I wasn’t entirely convinced. Although the paper is very good at describing the EDGES experiment, it is far less convincing that all necessary foregrounds and systematics have been properly accounted for. There are many artefacts that could mimic the signal shown in the diagram.

I went on to say

If true, the signal is quite a lot larger than amplitude than standard models predict. That doesn’t mean that it must be wrong – I’ve never gone along with the saying `never trust an experimental result until it is confirmed by theory’ – but it’s way too early to claim that it proves that some new exotic physics is involved. The real explanation may be far more mundane.

There’s been a lot of media hype about this result – reminiscent of the BICEP bubble – and, while I agree that if it is true it is an extremely exciting result – I think it’s far too early to be certain of what it really represents. To my mind there’s a significant chance this could be a false cosmic dawn.

I gather the EDGES team is going to release its data publicly. That will be good, as independent checks of the data analysis would be very valuable.

Well, there’s a follow-up paper that I missed when it appeared on the arXiv in May the abstract of which reads:

We have re-analyzed the data in which Bowman et al. (2018) identified a feature that could be due to cosmological 21-cm line absorption in the intergalactic medium at redshift z~17. If we use exactly their procedures then we find almost identical results, but the fits imply either non-physical properties for the ionosphere or unexpected structure in the spectrum of foreground emission (or both). Furthermore we find that making reasonable changes to the analysis process, e.g., altering the description of the foregrounds or changing the range of frequencies included in the analysis, gives markedly different results for the properties of the absorption profile. We can in fact get what appears to be a satisfactory fit to the data without any absorption feature if there is a periodic feature with an amplitude of ~0.05 K present in the data. We believe that this calls into question the interpretation of these data as an unambiguous detection of the cosmological 21-cm absorption signature.

You can read the full paper here (PDF). I haven’t kept up with this particular story, so further comments/updates/references are welcome through the box below!