Archive for Cosmology

Cosmology Talks: Julien Lesgourgues on Neutrino Masses

Posted in The Universe and Stuff with tags , , , on April 3, 2020 by telescoper

If you are missing your regular seminar experience because of the Coronavirus lockdown, Shaun Hotchkiss has set up a YouTube channel just for you!

The channel features technical talks rather than popular expositions so it won’t be everyone’s cup of tea but for those seriously interested in cosmology at a research level they should prove interesting.

Here’s an example in which Julien Lesgourgues talks about (not measuring neutrino masses with cosmological data.

Early Dark Energy and Cosmic Tension

Posted in The Universe and Stuff with tags , , , , , on March 19, 2020 by telescoper

To avoid talking any more about you-know-what I thought I would continue the ongoing Hubble constant theme. Rhere is an interesting new paper on the arXiv (by Hill et al.) about the extent to which a modified form of dark energy might relieve the current apparent tension.

The abstract is:

 

You can click on this to make it bigger; you can also download the PDF here.

I think the conclusion is clear and it may or may not be related to a previous post of mine here about the implications of Etherington’s theorem.

Here’s my ongoing poll on the Hubble constant poll. Feel free to while away a few seconds of your time working from home casting a vote!

 

 

Evidence for a Spatially Flat Universe?

Posted in The Universe and Stuff with tags , , , , , , on February 19, 2020 by telescoper

Yesterday I saw a paper by George Efstathiou and Steve Gratton on the arXiv with the title The Evidence for a Spatially Flat Universe. The abstract reads:

We revisit the observational constraints on spatial curvature following recent claims that the Planck data favour a closed Universe. We use a new and statistically powerful Planck likelihood to show that the Planck temperature and polarization spectra are consistent with a spatially flat Universe, though because of a geometrical degeneracy cosmic microwave background spectra on their own do not lead to tight constraints on the curvature density parameter ΩK. When combined with other astrophysical data, particularly geometrical measurements of baryon acoustic oscillations, the Universe is constrained to be spatially flat to extremely high precision, with ΩK = 0.0004 ±0.0018 in agreement with the 2018 results of the Planck team. In the context of inflationary cosmology, the observations offer strong support for models of inflation with a large number of e-foldings and disfavour models of incomplete inflation.

You can download a PDF of the paper here. Here is the crucial figure:

This paper is in part a response to a paper I blogged about here and some other related work with the same general thrust. I thought I’d mention the paper here, however, because it contains some interesting comments about the appropriate choice of priors in the problem of inference in reference to cosmological parameters. I feel quite strongly that insufficient thought is given generally about how this should be done, often with nonsensical consequences. It’s quite frustrating to see researchers embracing the conceptual framework of Bayesian inference but then choosing an inappropriate prior. The prior is not an optional extra – it’s one of the key ingredients. This isn’t a problem limited to the inflationary scenarios discussed in the above paper, by the way, it arises in a much wider set of cosmological models. The real cosmological flatness problem is that too many cosmologists use  flat priors everywhere!

 

PhD Studentship in Gravitational Wave Astrophysics at Maynooth University!

Posted in Uncategorized with tags , , , , on February 11, 2020 by telescoper

With the arrival of Dr John Regan in the Department of Theoretical Physics at Maynooth University we are delighted to announce a fully-funded PhD studentship. In order to boost the circulation, here’s a copy of the advert you can find on John’s own website.

–o–

 

Project Description. Recent detections of gravitational waves from stellar mass sized black holes with the LIGO observatory has opened up a new window for black hole astrophysics as well as heralding the dawn of multimessenger astrophysics. LIGO is sensitive to the mergers of black holes in the range 10 solar masses up to approximately 100 solar masses out to a few Megaparsecs.

LISA is the planned, next generation, space-based gravitational wave observatory due for launch in 2034. LISA will be sensitive to gravitational waves at a much lower frequency compared to LIGO and as a result will be able to detect the mergers of both much larger and much more distant black holes. Planning for LISA is now well underway and the science base and objectives are being determined.

This PhD project will involve computing gravitational wave forms from mergers of massive black holes from the early Universe – which will be detectable by LISA. The origin of massive black holes is currently unknown and hence being able to detect their mergers from the early Universe is seen as a critical aspect in understanding their formation pathways. In this project the student will use the state-of-the-art Enzo-E code to model the mergers of black holes. In doing so the student will be able to accurately compute the gravitational wave signal from black holes which are merging in the distant Universe thus making predictions for LISA.

Student fees and a full stipend (€18k per annum) are available as part of this studentship.

Candidate Criteria. Applicants should have (or be about to complete) an undergraduate degree and/or taught postgraduate degree in (applied) mathematics, (theoretical) physics, computer science or a related discipline. Past experience shows that successful applicants usually have a very good first class degree (or equivalent). Applicants with computational experience are particularly encouraged to apply. In addition, the applicants must have excellent communication, planning and team working skills.

Application Procedure

Application Deadline: Friday May 1st 2020

Students who wish to apply for this studentship should apply in writing to john.regan@mu.ie. Please put “PhD Studentship Position” in the subject of the email. The application must comprise:

  • A full CV
  • A covering letter outlining why you wish to pursue this PhD program
  • Two references, preferably from your current academic institution, outlining your suitability for the position

Shortlisted candidates will be notified of the outcome of the selection process in early May with interviews in mid-late May. The start date for the PhD is expected to be September 2020.

Please direct any questions or queries on the above position to Dr. John Regan (john.regan@mu.ie)

 

 

Voids, Galaxies and Cosmic Acceleration

Posted in The Universe and Stuff with tags , , , , , , on February 4, 2020 by telescoper

Time for a quick plug for a paper by Nadathur et al. that appeared on the arXiv recently with the title Testing low-redshift cosmic acceleration with large-scale structure. Here is the abstract:

You can make it bigger by clicking on the image. You can download a PDF of the entire paper here.

The particularly interesting thing about this result is that it gives strong evidence for models with a cosmological constant (or perhaps some other form of dark energy), in a manner that is independent of the other main cosmological constraints (i.e. the Cosmic Microwave Background or Type 1a Supernovae). This constraint is based on combining properties of void regions (underdensities) with Baryon Acoustic Oscillations (BAOs) to produce constraints that are stronger than those obtained using BAOs on their own. The data used derives largely from the BOSS survey.

As well as this there’s another intriguing result, or rather two results. First is that the the BAO+voids data from redshifts z<2 gives H0 = 72.3 ± 1.9, while, on the other hand adding, BAO information from the Lyman-alpha forest for from z>2 gives a value H0 = 69 \pm 1.2, favouring Planck over Riess. Once again, the `tension’ over the value of the Hubble constant appears to be related to using nearby rather than distant sources.

Luminosity Evolution in Type 1a Supernovae?

Posted in The Universe and Stuff with tags , , , , on January 14, 2020 by telescoper

Figure 1 of Kang et al.

During this afternoon’s very exciting Meeting of the Faculty of Science and Engineering at Maynooth University I suddenly remembered a paper I became aware of over Christmas but then forgot about. There’s an article here describing the paper that makes some pretty strong claims, which was what alerted me to it. The actual paper, by Kang et al., which has apparently been refereed and accepted for publication by the Astrophysical Journal, can be found on the arXiv here. The abstract reads:

The most direct and strongest evidence for the presence of dark energy is provided by the measurement of galaxy distances using type Ia supernovae (SNe Ia). This result is based on the assumption that the corrected brightness of SN Ia through the empirical standardization would not evolve with look-back time. Recent studies have shown, however, that the standardized brightness of SN Ia is correlated with host morphology, host mass, and local star formation rate, suggesting a possible correlation with stellar population property. In order to understand the origin of these correlations, we have continued our spectroscopic observations to cover most of the reported nearby early-type host galaxies. From high-quality (signal-to-noise ratio ~175) spectra, we obtained the most direct and reliable estimates of population age and metallicity for these host galaxies. We find a significant correlation between SN luminosity (after the standardization) and stellar population age at a 99.5% confidence level. As such, this is the most direct and stringent test ever made for the luminosity evolution of SN Ia. Based on this result, we further show that the previously reported correlations with host morphology, host mass, and local star formation rate are most likely originated from the difference in population age. This indicates that the light-curve fitters used by the SNe Ia community are not quite capable of correcting for the population age effect, which would inevitably cause a serious systematic bias with look-back time. Notably, taken at face values, a significant fraction of the Hubble residual used in the discovery of the dark energy appears to be affected by the luminosity evolution. We argue, therefore, that this systematic bias must be considered in detail in SN cosmology before proceeding to the details of the dark energy.

Of course evidence for significant luminosity evolution of Type Ia supernovae would throw a big spanner in the works involved in using these objects to probe cosmology (specifically dark energy), but having skimmed the paper I’m a bit skeptical about the results, largely because they seem to use only a very small number of supernovae to reach their conclusions and I’m not convinced about selection effects. I have an open mind, though, so I’d be very interested to hear through the comments box the views of any experts in this field.

Another Cosmologist for Maynooth!

Posted in Maynooth, The Universe and Stuff with tags , , on January 13, 2020 by telescoper

A few people have contacted me to ask what happened with the research position in cosmology at Maynooth University advertised a few months ago. Well, I am now in a position to provide the answer.

I’m very happy to announce that as of January 2nd 2020, Dr John Regan has joined the staff of the Department of Theoretical Physics, bringing with him an SFI – Royal Society University Research Fellowship (URF) which will fund his research for five years.

Dr John Regan

John’s primary area of research is in trying to understand the formation of black holes in the early Universe and their subsequent growth and evolution. He is interested in trying to determine how the first massive black holes in the Universe formed and the conditions required to form them. The problem is well posed since at early times the Universe was a comparatively simple place compared to the Universe today. Recent observations have indicated that Supermassive Black Holes existed less than 1 billion years after the big bang (the Universe is approximately 14 billion years old). A current open problem in Cosmology is how did black holes form and grow quickly enough in order to become super-massive so early in the Universe?

In answering this question John uses high resolution numerical simulations to study the environments in which the first massive black hole seeds may have formed and then grown to become the super-massive ones we can still observe today.

I’m delighted that John has joined the Department and look forward to many years of fruitful collaborations and discussions. He will be joined by a PDRA and a research student in due course.

You can follow John on Twitter here: