Archive for Cosmology

New Publication at the Open Journal of Astrophysics

Posted in Open Access, The Universe and Stuff with tags , , , , , , on January 27, 2023 by telescoper

Time to announce another new paper at the Open Journal of Astrophysics. This one was published yesterday, 26th January 2023. The latest paper is the third paper in Volume 6 (2023) as well as the 68th in all. It’s yet another in the Cosmology and NonGalactic Astrophysics folder.

The latest publication is entitled “Palatini formulation for gauge theory: implications for slow-roll inflation” and the authors are Syksy Räsänen of the University of Helsinki in Finland and Yosef Verbin (The Open University of Israel, Ra’anana, Israel). The first author has  published a previous paper on the Palatini formulation in the Open Journal of Astrophysics.

Here is a screen grab of the overlay which includes the  abstract:

 

 

You can click on the image to make it larger should you wish to do so. You can find the officially accepted version of the paper on the arXiv here.

P.S. You may be wondering about the image shown in the overlay. This paper doesn’t contain any figures or images so I tried out the collection of stock photographs that comes free with the Scholastica platform by typing in “gauge”. The result was a quite amusing collection of pictures of various kinds of dials and other gauges. I quite liked the one above so used it just for show!

New Publication at the Open Journal of Astrophysics

Posted in Open Access, The Universe and Stuff with tags , , , , on January 7, 2023 by telescoper

Continuing the process of catching up with business at the Open Journal of Astrophysics, here is the first paper of 2023. This one was accepted before Christmas but the final version only appeared on arXiv after the holiday and was published officially on 4th January 2023.

The latest paper is the first paper in Volume 6 (2023) as well as the 66th in all. It’s yet another in the Cosmology and Non-Galactic Astrophysics folder.

The latest publication is entitled “It takes two to know one: Computing accurate one-point PDF covariances from effective two-point PDF models“. This is a British-French-German collaboration led by Cora Uhlemann of Newcastle University with co-authors  Oliver Friedrich, Aoife Boyle, Alex Gough, Alexandre Barthelemy, Francis Bernardeau, and Sandrine Codis.

This is such an interesting paper that we discussed it at our cosmology journal club at Maynooth University a while ago when it first appeared on arXiv and reading it again since then has suggested a nice project to me!

Anyway, here is a screen grab of the overlay which includes the  abstract:

 

 

You can click on the image to make it larger should you wish to do so. You can find the officially accepted version of the paper on the arXiv here.

New Publication at the Open Journal of Astrophysics

Posted in Open Access, The Universe and Stuff with tags , , on January 5, 2023 by telescoper

I’ve been catching up on publishing matters over the past day or so, including dealing with a bit of a backlog generated by the Christmas break. The Open Journal is run entirely by volunteers and we all need some time off at some point.

To start with I’m delighted to be able to announce the last paper of 2022 at the Open Journal of Astrophysics.  The latest paper is the 17th paper in Volume 5 (2022) as well as the 65th in all. It’s yet another in the Cosmology and Non-Galactic Astrophysics folder.

The latest publication is entitled “The Cosmic Graph: Optimal Information Extraction from Large-Scale Structure using Catalogues“. It is written by a distinguished collection of cosmologists from around the world (and Alan Heavens).

Anyway, here is a screen grab of the overlay which includes the  abstract:

 

You can click on the image to make it larger should you wish to do so. You can find the officially accepted version of the paper on the arXiv here.

Here is a bigger version of the image from the paper used on the overlay:

 

 

Code and a tutorial for the analysis and relevant software can be found here .

Latest Results from the South Pole Telescope

Posted in The Universe and Stuff with tags , , , , on December 13, 2022 by telescoper

Just time for a quick post to point out the latest results from the South Pole Telescope (SPT) have now hit the arXiv. The measurements were made in 2018 but the outcome of a full analysis of temperature and polarization has only just appeared.

Here’s a grab of the abstract:

The key figures showing the constraints on the Hubble Constant H0 and the parameter S8 are shown here:

As you can see, the results from SPT-3G are consistent with the standard cosmological model and agree on H0 with Planck rather than the higher value obtained from local measurements. If you thought there was Hubble tension before this measurement, then you will still think so now!

What is a Singularity?

Posted in Education, Maynooth, The Universe and Stuff with tags , , , , , , on November 24, 2022 by telescoper

Following last week’s Maynooth Astrophysics and Cosmology Masterclass, a student asked (in the context of the Big Bang or a black hole) what a singularity is. I thought I’d share my response here in case anyone else was wondering. The following is what I wrote back to my correspondent:

–oo–

In general, a singularity is pathological mathematical situation wherein the value of a particular variable becomes infinite. To give a very simple example, consider the calculation of the Newtonian force due  to gravity exerted by a massive body on a test particle at a distance r. This force is proportional to 1/r2,, so that if one tried to calculate the force for objects at zero separation (r=0), the result would be infinite.

Singularities are not always  signs of serious mathematical problems. Sometimes they are simply caused by an inappropriate choice of coordinates. For example, something strange and akin to a singularity happens in the standard maps one finds in an atlas. These maps look quite sensible until one looks very near the poles.  In a standard equatorial projection,  the North Pole does not appear as a point, as it should, but is spread along straight line along the top of the map. But if you were to travel to the North Pole you would not see anything strange or catastrophic there. The singularity that causes this point to appear is an example of a coordinate singularity, and it can be transformed away by using a different projection.

More serious singularities occur with depressing regularity in solutions of the equations of general relativity. Some of these are coordinate singularities like the one discussed above and are not particularly serious. However, Einstein’s theory is special in that it predicts the existence of real singularities where real physical quantities (such as the matter density) become infinite. The curvature of space-time can also become infinite in certain situations.

Probably the most famous example of a singularity lies at the core of a black hole. This appears in the original Schwarzschild interior solution corresponding to an object with perfect spherical symmetry. For many years, physicists thought that the existence of a singularity of this kind was merely due to the special and rather artificial nature of the exactly spherical solution. However, a series of mathematical investigations, culminating in the singularity theorems of Penrose, showed no special symmetry is required and that singularities arise in the generic gravitational collapse problem.

As if to apologize for predicting these singularities in the first place, general relativity does its best to hide them from us. A Schwarzschild black hole is surrounded by an event horizon that effectively protects outside observers from the singularity itself. It seems likely that all singularities in general relativity are protected in this way, and so-called naked singularities are not thought to be physically realistic.

There is also a singularity at the very beginning in the standard Big Bang theory. This again is expected to be a real singularity where the temperature and density become infinite. In this respect the Big Bang can be thought of as a kind of time-reverse of the gravitational collapse that forms a black hole. As was the case with the Schwarzschild solution, many physicists thought that the initial cosmologcal singularity could be a consequence of the special symmetry required by the Cosmological Principle. But this is now known not to be the case. Hawking and Penrose generalized Penrose’s original black hole theorems to show that a singularity invariably exists in the past of an expanding Universe in which certain very general conditions apply.

So is it possible to avoid this singularity? And if so, how?

It is clear that the initial cosmological singularity might well just be a consequence of extrapolating deductions based on the classical ttheory of general relativity into a situation where this theory is no longer valid.  Indeed, Einstein himself wrote:

The theory is based on a separation of the concepts of the gravitational field and matter. While this may be a valid approximation for weak fields, it may presumably be quite inadequate for very high densities of matter. One may not therefore assume the validity of the equations for very high densities and it is just possible that in a unified theory there would be no such singularity.

Einstein, A., 1950. The Meaning of Relativity, 3rd Edition, Princeton University Press.

We need new laws of physics to describe the behaviour of matter in the vicinity of the Big Bang, when the density and temperature are much higher than can be achieved in laboratory experiments. In particular, any theory of matter under such extreme conditions must take account of  quantum effects on a cosmological scale. The name given to the theory of gravity that replaces general relativity at ultra-high energies by taking these effects into account is quantum gravity, but no such theory has yet been constructed.

There are, however, ways of avoiding the initial singularity in classical general relativity without appealing to quantum effects. First, one can propose an equation of state for matter in the very early Universe that does not obey the conditions laid down by Hawking and Penrose. The most important of these conditions is called the strong energy condition: that r+3p/c2>0 where r is the matter density and p is the pressure. There are various ways in which this condition might indeed be violated. In particular, it is violated by a scalar field when its evolution is dominated by its vacuum energy, which is the condition necessary for driving inflationary Universe models into an accelerated expansion.  The vacuum energy of the scalar field may be regarded as an effective cosmological constant; models in which the cosmological constant is included generally have a bounce rather than a singularity: running the clock back, the Universe reaches a minimum size and then expands again.

Whether the singularity is avoidable or not remains an open question, and the issue of whether we can describe the very earliest phases of the Big Bang, before the Planck time, will remain open at least until a complete  theory of quantum gravity is constructed.

Astrophysics & Cosmology Masterclass Next Week!

Posted in Maynooth, The Universe and Stuff with tags , , , , on November 11, 2022 by telescoper

As next week is Science Week I thought I’d remind readers that as part of the festivities we are hosting a virtual  Masterclass in Astrophysics & Cosmology in Maynoothon Wednesday 16th November 2022  .

You may remember that we have presented such events twice before. Last year’s event was a particularly big success, with over a hundred schools joining in, with probably over a thousand young people listening and asking questions.

Like last year’s event this year’s will be a half-day virtual event via Zoom. It’s meant for school students in their 5th or 6th year of the Irish system. There might be a few of them or their teachers who see this blog so I thought I’d share the news here. You can find more information, including instructions on how to book a place, here.

Here is the flyer for the event:

I’ll be talking about cosmology early on, and John Regan will talk about black holes later on. After the coffee break one of our students will talk about why they wanted to study astrophysics. Then I’ll say something about our degree programmes for those students who might be interested in studying astrophysics and/or cosmology as part of a science course. We’ll finish with questions either about the science or the studying!

Here is a more detailed programme:

Astrophysics & Cosmology Masterclass at Maynooth

Posted in Education, Maynooth, The Universe and Stuff with tags , , , on October 24, 2022 by telescoper

Regular readers of the blog – both of them – may remember that we have twice previously presented a Masterclass in Astrophysics & Cosmology in Maynooth. Well, owing to popular demand, we’ve decided to do a re-run of the event on Wednesday 16th November 2022 during this year’s Science Week. Last year’s event was a big success, with over a hundred schools joining in, with probably over a thousand young people listening and asking questions.

Like last year’s event this year’s will be a half-day virtual event via Zoom. It’s meant for school students in their 5th or 6th year of the Irish system. There might be a few of them or their teachers who see this blog so I thought I’d share the news here. You can find more information, including instructions on how to book a place, here.

Here is the flyer for the event:

I’ll be talking about cosmology early on, and John Regan will talk about black holes later on. After the coffee break one of our students will talk about why they wanted to study astrophysics. Then I’ll say something about our degree programmes for those students who might be interested in studying astrophysics and/or cosmology as part of a science course. We’ll finish with questions either about the science or the studying!

Here is a more detailed programme:

Fortunately this year I don’t need to dash away at noon to do a lecture!

SpaceX for Euclid

Posted in The Universe and Stuff with tags , , , , , , , on October 20, 2022 by telescoper

Aa few months ago I posted a piece about the European Space Agency’s Euclid Mission which had been due to be launched in 2023 on a Soyuz ST 2-1b rocket. That no longer being possible because of Russian’s invasion of Ukraine, it seemed there would be a lengthy delay in the launch of Euclid, with late 2024 seeming the earliest feasible date for launch on the obvious alternative, the new Ariane 6.

I ended that piece with this:

It seems to me that the best hope for a resolution of this problem would be for ESA to permit the launch of Euclid using something other than Ariane 6, which means using a vehicle supplied by an independent commercial operator. I sincerely hope ESA is able to come up with an imaginative solution to this very serious problem.

In the Dark, 17th June

Well I’ve just read official confirmation that a few hours ago ESA Council has approved the proposal to launch Euclid on a Falcon 9 rocket operated by SpaceX. If all goes well -specifically if the Euclid Consortium member states agree with this move – it might be possible to launch Euclid by the end of 2023. Although I don’t have any firm information about what date is being proposed I believe it could be as soon as July 2023.

Setting aside any personal opinions about Elon Musk, the Falcon 9 has proved to be very reliable, with the latest version having 110 out of 110 successful launches. Euclid will not be in an Earth orbit, like most of the satellites so far launched by SpaceX, but has to be delivered to the 2nd Lagrange Point, L2. That should not pose to much of a difficulty, however.

That Was The (Space) Week That Was

Posted in Biographical, Maynooth, The Universe and Stuff with tags , , , , on October 7, 2022 by telescoper

Last night I participated in an event at Maynooth for Space Week which I think went very well. We had a big audience so the decision to move to a bigger lecture theatre was a good one. Nobody took count but I think we had as many as 400 people of all ages, including some very young kids, some students and a variety of others.

I was the last one up to speak and took a few pictures at the three talks before mine but obviously couldn’t take a picture of mine so I’ve included a pic of some of the hi-tech equipment I used for a couple of demonstrations:

If anyone wants to see the pictures I showed you can find them here:

There was an official photographer there last night so I’ll upload any pictures I come across in due course. Watch this space.

UPDATE: Here’s a picture of the four speakers

Last night’s four speakers: Créidhe O’Sullivan, Me, Emma Whelan and John Regan

Anyway, thank you to everyone for coming last night and especially to all the people who helped organize and run the event, including our student volunteers. We’re planning to do similar event for space week next year and hopefully this will become a regular feature in the calendar.

“New” Publication at the Open Journal of Astrophysics

Posted in Open Access, The Universe and Stuff with tags , , , , , on September 15, 2022 by telescoper

It’s time once again for me to announce the publication of another paper at the Open Journal of Astrophysics. The new paper, published last week, is the 14th paper in Volume 5 (2022) and the 62nd in all. The latest publication is entitled “Gravitational Stability of Vortices in Bose-Einstein Condensate Dark Matter”. This paper is another one for the folder marked Cosmology and Non-Galactic Astrophysics and the authors are Mark N Brook Now at the Institute for Cancer Research in London) and Peter Coles (Who he? Ed).

There is a bit of a story behind this one. The work on which this paper is based was done while both authors (Mark and I) were at the University of Nottingham. Mark was my PhD student at the time.  I left Nottingham for Cardiff in 2007 but Mark stayed behind to finish his thesis and write this paper, which appeared on the arXiv in 2009. The paper wasn’t accepted in its original form, Mark left the field after obtaining his PhD, and I was working on other things at Cardiff so the paper remained unpublished on the arXiv.

Last year, however, I was updating my publication list and noticed the old preprint so looked it up on NASA/ADS. Although not Earth-shattering, I found it had been acquiring a reasonable number of citations (16 according to ADS, including some this year) as an unpublished work largely because of increased interest in the field of condensate dark matter. I therefore approached the Editorial Board of the Open Journal of Astrophysics to ask their opinion about whether it would be appropriate to consider it for publication. They agreed and the paper was assigned to an Editor. Obviously I recused myself from the process.

Somewhat to my surprise, given that it’s basically an old paper, the referee comments were supportive. I’ve been very busy for the past year and communication with Mark was slow so it’s taken a while to revise and update the paper in line with the referee requests. We also took the opportunity to include a brief review of some papers that had come out since the original version of the paper appeared. Mark and I agreed a final text l and the paper was accepted last week. I uploaded the agreed version to arXiv and now the paper is now published. It was all a bit unconventional but there we are. It was interesting to be on the author side of the process for a change!

Anyway, here is a screen grab of the overlay which includes the  abstract:


You can click on the image to make it larger should you wish to do so. You can find the accepted version of the paper on the arXiv here.