Archive for Cosmology

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Hubble Tension Reviewed

Posted in The Universe and Stuff with tags , , on November 24, 2023 by telescoper

Just a quick post to pass on a reference to a paper on arXiv (to appear in Annual Reviews of Astronomy and Astrophysics) about the ongoing saga of the Hubble Tension. The authors are Licia Verde, Nils Schöneberg, and Héctor Gil-Marín, three members of the ICCUB which is hosting me during my sabbatical. I saw an earlier draft of this paper but didn’t want to blog about it before the final version appeared. The abstract (which I’ve slightly reformatted) reads:

The Hubble parameter H0, is not a univocally-defined quantity: it relates redshifts to distances in the near Universe, but is also a key parameter of the ΛCDM standard cosmological model. As such, H0 affects several physical processes at different cosmic epochs, and multiple observables. We have counted more than a dozen H0‘s which are expected to agree if a) there are no significant systematics in the data and their interpretation and b) the adopted cosmological model is correct. With few exceptions (proverbially confirming the rule) these determinations do not agree at high statistical significance; their values cluster around two camps: the low (68 km/s/Mpc) and high (73 km/s/Mpc) camp. It appears to be a matter of anchors: the shape of the Universe expansion history agrees with the model, it is the normalizations that disagree. Beyond systematics in the data/analysis, if the model is incorrect there are only two viable ways to “fix” it: by changing the early time (z≳1100) physics and thus the early time normalization, or by a global modification, possibly touching the model’s fundamental assumptions (e.g., homogeneity, isotropy, gravity). None of these three options has the consensus of the community. The research community has been actively looking for deviations from ΛCDM for two decades; the one we might have found makes us wish we could put the genie back in the bottle.

arXiv:2311.13305


You can read the full paper here to learn about the scientific arguments, but I’d like to draw attention to this excerpt which is of more general relevance and with which I agree wholeheartedly:

It is also fair to say that the developments of the last decade have changed the expectations and modus operandi of a big part of the community. The community now expects results to be reproducible, hence the data and key software to be publicly available in such a way that a practitioner not involved in the original analysis could still retrace and reproduce all important steps and findings. While research areas such as the CMB and large-scale structure made this transition to “open science” about two decades ago, this was not the case for other areas of extra-galactic astronomy, but this is now changing.

arXiv:2311.13305

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Euclid, the Perseus Cluster, and Beyond!

Posted in Euclid, The Universe and Stuff with tags , , , on November 8, 2023 by telescoper

I see that the Euclid Early Release Observation of the Perseus Cluster is today’s Astronomy Picture of the Day:

The Perseus Cluster seen by Euclid

The Perseus Cluster (Abell 426) – a dense concentration of over a thousand galaxies with a total mass of about 1.2 × 1015 M⊙ – is impressive in its own right, especially because the picture was taken in a single exposure, but the staggering thing about this image is that it contains hundreds of thousands of galaxies. In other words there are as many galaxies in this picture as there are words in a book. Most of these galaxies are in the background, not associated with the cluster, and many of them extremely distant. With so many objects in one field, you can perhaps see how much data we will get from the entire survey, which will last more than 6 years. It is these distant sources – billions of them – that Euclid will survey to glean information about the expansion history of the Universe.

P.S. Yesterday’s press conference resulted in a lot of media attention worldwide, even some in Ireland. There is a piece on the RTÉ website, for example, and another on Silicon Republic (which includes a quote from yours truly).

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Euclid Update!

Posted in Euclid with tags , , , on November 6, 2023 by telescoper

Just one more sleep before the first actual science images from European Space Agency’s Euclid mission are released tomorrow. These are called the Early Release Observations (EROs) – they aren’t part of the full survey, but are to demonstrate the performance of the telescope and detectors.

There has been a slight change to the schedule advertised here so the press conference will now take place on Tuesday 7th November at 14.15 Central European (not Summer) Time, CET, which is 13.15 GMT. For more details, see here.

You can watch the press conference on the new ESA Web TV channel or on the Youtube Live Stream. I’ll post a quick update tomorrow after the images go public.

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Preparing for Euclid’s First Images

Posted in Euclid, The Universe and Stuff with tags , , , on November 3, 2023 by telescoper

Another quick update about the release of the Early Release Observations (EROs) from Euclid, due to take place next Tuesday 7th November. For one thing, here is a little taster video.

Five images will be released on Tuesday. I know what the Early Release Observations are but you will have to wait until Tuesday to find out. If I told you now I’d have to kill you…

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Euclid Update!

Posted in Euclid with tags , , , on October 30, 2023 by telescoper

Just a quick post to give advanced notice that, gremlins in the pointing system having been dispelled, the first actual science images from European Space Agency’s Euclid mission will be released on Tuesday 7th November at 14.00 Central European (not Summer) Time, CET. These are called the Early Release Observations (EROs) – they won’t be part of the full survey, but are just to demonstrate the performance of the telescope and detectors.

You can watch the press conference on the new ESA Web TV channel. I’ll post more about the EROs after they become public, but not before.

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New Publication at the Open Journal of Astrophysics

Posted in OJAp Papers, Open Access, The Universe and Stuff with tags , , , , , on October 22, 2023 by telescoper

It’s Sunday but I’ll be a bit busy next week so I’m taking the opportunity today to announce yet another new paper at the Open Journal of Astrophysics. This one was published on Friday 20th October.

The latest paper is the 41st  so far in Volume 6 (2023) and the 106th in all. It is a product of the Dark Energy Survey team and the Kilo-Degree Survey Collaboration, which amounts to about 160 authors altogether. The corresponding author for this article was the Astronomer Royal for Scotland Professor Catherine Heymans, no less.

The primary classification for this paper is Cosmology and NonGalactic Astrophysics and its title is “DES Y3 + KiDS-1000: Consistent cosmology combining cosmic shear surveys”. The article presents a joint analysis of the Dark Energy Survey Year 3 data and the Kilo-Degree Survey data, with a discussion of the implications for cosmological parameters. The key figure – a very important one – is this:

If you want to know more about the result and why it is so important you could read the paper. It is, however, rather long: 40 pages including 21 figures and 15 tables. Do not despair, though, because here is a video explaining the work in the series of Cosmology Talks presented by Shaun Hotchkiss:

Anyway, here is the overlay of the paper containing the abstract:

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

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Two New Publications at the Open Journal of Astrophysics

Posted in OJAp Papers, Open Access, The Universe and Stuff with tags , , , , , , , , on October 20, 2023 by telescoper

It’s Friday so it’s a good time to catch up with the week’s action at the Open Journal of Astrophysics, where there have been two new publications so far this week. These papers take us up to a total of 40  in Volume 6 (2023) and 105 in total since we started publishing.

The title of the first paper is “Halo Properties from Observable Measures of Environment: I. Halo and Subhalo Masses” and its primary classification is Astrophysics of Galaxies. it is an exploration using neural networks of how the peak masses of dark matter halos and subhaloes correlate with observationally-accessible measures of their dependence on environment.

The authors based in the United States of America: Haley Bowden and Peter Behroozi of the University of Arizona, and Andrew Hearin of the Argonne National Laboratory

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

 

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

The second paper was published on 18th October 2023.  The primary classification for this one is Cosmology and Nongalactic Astrophysics and is “Mitigating the noise of DESI mocks using analytic control variates”. For those of you not up with the lingo, DESI stands for the Dark Energy Spectroscopic Instrument and you can read more about it here.

The lead author for this one is Boryana Hadzhiyska of the Lawrence Berkeley Laboratory and the University of California, Berkeley (USA) and there are 32 other authors. This paper presents a method for reducing the effects of sample variance on cosmological simulations using analytical approximations and tests it using DESI data.

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

 

 

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

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The Return of a Small Universe?

Posted in The Universe and Stuff with tags , , , , on October 5, 2023 by telescoper

Today I attended a cosmology discussion group where the paper being considered was by Jean-Luc Lehners and Jerome Quintin and was entitled A small Universe. Here is the abstract:

Many cosmological models assume or imply that the total size of the universe is very large, perhaps even infinite. Here we argue instead that the universe might be comparatively small, in fact not much larger than the currently observed size. A concrete implementation of this idea is provided by the no-boundary proposal, in combination with a plateau-shaped inflationary potential. In this model, opposing effects of the weighting of the wave function and of the criterion of allowability of the geometries conspire to favour small universes. We point out that a small size of the universe also fits well with swampland conjectures, and we comment on the relation with the dark dimension scenario.

arXiv:2309.03272

This paper is based on rather speculative arguments. I don’t have anything against those, but the discussion of this particular case reminded me that the idea that the Universe might be much smaller than we think is one that has come and gone many times during my lifetime. The point is that Einstein’s equations of general relativity are local in that they relate the geometric properties of space-time at specific coordinate position to the energy and momentum at the same position. When we make cosmological models based on these equations we usually assume a great deal of symmetry, i.e. that defined in a certain way the spatial sections which form surfaces of simultaneity have the same curvature everywhere, regardless of spatial position. The standard cosmology takes this curvature to be zero, in fact, so the spatial sections are Euclidean (flat), though the curvature could be positive or negative.

Usually when we assume the universe is flat we also assume that it is infinite, but it is possible in principle that a flat universe could be finite, for example in the case of a cube with opposite faces identified so that it has a sort of toroidal symmetry that has no physical edge. The size of the notional cube defines a topological scale which is independent of Einstein’s equations. That’ just a simple example: the topology does not have to be based on a cube; it could be, for example, a rhombic dodecahedron…

Likewise when we talk about a universe with negative spatial curvature we also assume it to be infinite, but that doesn’t have to be the case either: there are spaces with negative spatial curvature which are finite. A manifestation of this idea that I remember from way back in 1999 was a paper by Neil Cornish and David Spergel entitled A small universe after all?

Observing a small universe from the inside produces many interesting effects like a sort of cosmic hall of mirrors. For instance, if you can see far enough you will see the back of your own head. More realistically, the observed large-scale structure of the universe would repeat, and there would be correlated features in the cosmic microwave background. The idea is therefore amenable to observational test; the absence of any of the predicted correlations places constrains the topological scale to be comparable to the size of the observable universe or larger. Of course if it’s infinitely large then the small universe is not small at all…

(For a bit of gratuitous self-promotion, I refer you to a paper I wrote with Graca Rocha and Patrick Dineen back in 2004 using the WMAP observations of the CMB to constrain compact topologies. Given the wealth of new data we have acquired since then I’m sure the constraints are even stronger now.)

Anyway, my point is that speculative ideas are all very well but they don’t mean much if you can’t test them. This one at least has the virtue of making testable predictions.

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New Publication at the Open Journal of Astrophysics

Posted in OJAp Papers, Open Access, The Universe and Stuff with tags , , , on September 28, 2023 by telescoper

Time to announce yet another new paper at the Open Journal of Astrophysics. This one was published yesterday (27th September 2023).

The latest paper is the 37th  so far in Volume 6 (2023) and the 102nd in all. The authors are Joe McCaffrey (Maynooth, Ireland), Samantha Hardin (Georgia Tech, USA), John Wise (Georgia Tech) and John Regan (Maynooth). As this one involved two authors from my own Department, I recused myself from the editorial process, although it is work I am very interested in.

The primary classification for this paper is Astrophysics of Galaxies and its title is “No Tension: JWST Galaxies at z>10 Consistent with Cosmological Simulations”.  I’ve blogged about this paper before, a few months ago, when it appeared on the arXiv. The editorial process on this one has been very thorough and  it has been a few rounds with the reviewers before being accepted for publication. The authors may have found this a bit irksome, but I think the process improved them paper considerably, which is what it is meant to do.

As many of you will be aware, there’s been a considerable to-do not to mention a hoo-hah about the detections by JWST of some galaxies at high redshift. Some of these have been shown not to be galaxies at high redshift after all, but some around z=10 seem to be genuine. This paper is a response to claims that these somehow rule out the standard cosmological framework.

The key figure in the current paper is this:

The solid curves show the number of galaxies of a given mass one would expect to see as a function of redshift in fields comparable to those observed with estimated values from observations (star-shaped symbols). As you can see the observed points are consistent with the predictions. There’s no tension, so you can all relax.

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

 

 

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

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The Universe from Einstein to Euclid

Posted in Biographical, Talks and Reviews with tags , , on September 7, 2023 by telescoper

As we head into the Day 2 of the ITP2023 I thought I’d share the slides I used for the public talk I gave last night. We had an audience of around a hundred which wasn’t bad given that it is graduation week and the undergraduates aren’t back!

Here is the abstract used to advertise the talk:

Euclid is the name of a new scientific mission from the European Space Agency, launched on July 1st, designed to explore the composition and evolution of the Universe. The Euclid mission takes its name from the ancient Greek mathematician regarded by many as the Father of geometry. Until the last century, Euclid’s theorems were assumed not just to be mathematical notions, but to describe the geometrical structure of the physical Universe. Einstein’s general theory of relativity swept that idea aside and gave us new ways of describing space, by unifying it with time, and by allowing it to be affected by matter in a manner very different from that formulated by Euclid. Over the past century, this theory has proved to be very effective at describing the properties of the Universe as observed by modern astronomical telescopes, while also suggesting the existence of dark matter and dark energy.

The Euclid telescope will create an enormous map of the large-scale structure of the Universe across space and time by observing billions of galaxies out to 10 billion light-years, across more than a third of the sky. Euclid will explore how the Universe has expanded and how galaxies and clusters of galaxies have formed over cosmic history, and how space itself is distorted by these structures.

This talk will discuss our modern ideas of space and time, how the Euclid mission will try to test whether or not they are correct and shed light on the nature of dark matter and dark energy.

And here are the slides:

Obviously I cut a number of long stories very short, which probably contributed to why I had a lot of questions from the audience at the end of the talk. I always assume that’s a basically a good sign because it shows people are interested, but it also makes me worry that I didn’t explain things very well!

We didn’t finish until past 9 o’clock and it was a very warm evening, so I was very happy to have a few pints afterwards in O’Neills…

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