Archive for Cosmology

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:

New Publication at the Open Journal of Astrophysics!

Posted in Open Access, The Universe and Stuff with tags , , , on January 7, 2020 by telescoper

Back after the New Year break we have published another new paper at The Open Journal of Astrophysics, our first of 2020!

Here is a grab of the overlay:

The author is Phillip Helbig (Who? Ed). This is a review article. Our `For Authors‘ page includes the following:

We may accept review articles on appropriate subjects if the Editorial Board considers them a useful contribution to the literature.

You can find the accepted version on the arXiv here. This is another one for the `Cosmology and Nongalactic Astrophysics’ section . We would be very happy to get more submissions from other areas, especially Stellar and Planetary astrophysics. Hint! Hint!

P.S. Just another reminder that we now have an Open Journal of Astrophysics Facebook page where you can follow updates from the Journal should you so wish..

With the Cosmic Web in Mind..

Posted in Astronomy Lookalikes, The Universe and Stuff with tags , , , , , on November 23, 2019 by telescoper

Some time ago I posted one of my Astronomy Look-alikes about the remarkable similarity between the structure of the human brain and that revealed by computer simulations of the large-scale structure of the Universe:

I wonder whether this means that the Cosmic Web is really just all in the mind?

Anyway I just came across an article by Franco Vazza and Alberto Fenetti that takes the comparison between brain cells (among other things) and the Cosmic Web a bit further, including a look at the corresponding power spectra:

The main point to take from this picture is that many naturally occurring patterns have approximately power-law power spectra, at least over a limited range of scales. However, as I have pointed out before on this blog, the power spectrum on its own does not really quantify pattern in any meaningful way. Here for example are two patterns with exactly the same power spectrum:

The point is that the power spectrum does not contain any information about the phase correlations of the Fourier modes, which are important in quantifying localised features. For further discussion of this issue, see here.

Cosmology Job Opportunity at Maynooth!

Posted in Maynooth with tags , on November 11, 2019 by telescoper

With everything else going on I completely forgot to mention on here that there is a job opportunity for a senior research position in cosmology here in the Department of Theoretical Physics here in Maynooth that may
lead to a permanent position subject to satisfactory performance. The position was advertised some time ago through the usual channels (including jobs.ac.uk and the Times Higher job site) as well as on the Maynooth University jobs site.

The details can be found here, at which site you can also apply.

You will have to hurry, though, as the deadline is Wednesday 13th November 2020 – about 48 hours from now!

First Light at the Dark Energy Spectroscopic Instrument

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

While I was away last week there was quite a lot of press coverage (e.g. here) about the new Dark Energy Spectroscopic Instrument, which has just seen first light. I didn’t have time to mention this until now, and in any case  I have little to add to the coverage that has already appeared, but it does give me the excuse to post this nice video – which features quite a few people I actually know! – to describe  the huge galaxy survey that DESI will perform. It’s hard to believe that when I started in the field in 1985 the largest such survey, which took several years to compile, had only a few thousand galaxies in it. The DESI instrument will be able to determine spectra of more sources than that in a single pointing of the telescope that lasts about 20 minutes. Overall it should determine redshifts of over 35 million galaxies! Vorsprung durch Technik.

 

 

A Nobel Prize for Jim Peebles!

Posted in The Universe and Stuff with tags , , , , , , on October 8, 2019 by telescoper

I’ve just dashed back in excitement to the office from two hours of mandatory Financial Report Training to write a quick post before my 12 o’clock lecture on Astrophysics & Cosmology because of the news about the award of the 2019 Nobel Prize for Physics.

My recent post was half right in the sense that half this year’s prize goes to Michel Mayor and Didier Queloz for the discovery of an extrasolar planet. I don’t know either of them personally, but heartiest congratulations to both!

My heart lept with joy, however, to see the other half of the prize go to Jim Peebles (above) for his work on theoretical cosmology. Much of the reason for that is that I’ve had the great honour and pleasure to meet Jim many times over the years. He is not only a truly great scientist but also a extremely nice man whose kindness and generosity is universally recognized. He’s not known as `Gentleman Jim’ for nothing!

The other reason for the excitement is that I was completely taken by surprise by the announcement. I had feared that his chance of winning a Nobel Prize had passed – I argued at the time that Jim should have been awarded a share of the 2006 Nobel Prize because without his amazing pioneering theoretical work the importance of the cosmic microwave background for cosmology and the large-scale structure of the Universe would not have been established so rapidly. As an author of the first paper to provide a theoretical interpretation of the signal detected by Penzias and Wilson, Jim was there right at the start of the modern era of cosmology and his subsequent work constructed the foundations of the theory of structure formation through gravitational instability. I was sad that he didn’t get a share in 2006 for this work, but am absolutely delighted that this has been rectified now!

This was one of the first cosmology books I ever bought. It’s an amazing piece of work that has been essential reading for cosmologists for almost 40 years!

Congratulations to Jim!

Now let me think about what to say to my students about this!