Archive for BOSS

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.

A Dark Expletive

Posted in Poetry, The Universe and Stuff with tags , , , , , , , on November 13, 2012 by telescoper

A news item today about BOSS (yet another observational cosmology survey) gives me an excuse to recycle an idea from an old post.

The phrase expletive deleted was made popular at the time of Watergate after the release of the expurgated tapes made by Richard Nixon in the Oval Office when he was President of the United States of America. These showed that, as well as been a complete crook, he was practically unable to speak a single sentence without including a swear word.

Nowadays the word expletive is generally taken to mean an oath or exclamation, particularly if it is obscene, but that’s not quite what it really means. Derived from the latin verb explere (“to fill out”) from which the past participle is expletus, the meaning of the word in the context of English grammar is  “something added to a phrase or sentence that isn’t strictly needed for the grammatical sense”.  An expletive is added either to fill a syntactical role or, in a poem, simply to make a line fit some metrical rule.

Examples of the former can be found in constructions like “It takes two to Tango” or “There is a lot of crime in Nottingham”; neither  “it” nor “there” should really be needed but English just seems to like to have something before the verb.

The second kind of use is illustrated wonderfully by Alexander Pope in his Essay on Criticism, which is a kind of guide to what to avoid in writing poetry. It’s a tour de force for its perceptiveness and humour. The following excerpt is pricelessly apt

These equal syllables alone require,
Tho’ oft the open vowels tire;
While expletives their feeble aid do join;
And ten low words oft creep in one dull line

Here the expletive is “do”,  and it is cleverly incorporated in the line talking about expletives, adding  the syllable needed to fit with a strict pentameter. Apparently, poets often used this construction before Pope attacked it but it quickly fell from favour afterwards.

His other prosodic targets are the “open vowels” which means initial vowels that produce an ugly glottal sound, such as in “oft” (especially ugly when following “Tho”). The last line is brilliant too, showing how using only monosyllabic “low” words makes for a line that plods along tediously just like it says.

It’s amazing how much Pope managed to fit into this poem, given the restrictions imposed by the closed couplet structure he adopted. Each idea is compressed into a unit of twenty syllables, two lines of ten syllables with a rhyme at the end of each. This is such an impressive exercise in word-play that it reminds me a lot of the skill showed by the best cryptic crossword setters. Needless to say I’m no more successful at writing poetry than I am at setting crossword clues.

Anyway, what’s all this got to do with cosmology?

Well, I was reminded of it when I attended the 2012 Gerald Whitrow Lecture by Andrew Liddle last Friday at the Royal Astronomical Society, during which he talked, amongst other things, about Dark Energy.

The Dark Energy is an ingredient added to the standard model of cosmology to reconcile  observations of a flat Universe with a matter density that seems too low to account for it.

Other than that it makes the  cosmological metric work out satisfactorily (geddit?), we don’t understand what Dark Energy really is  or why there is as much of it. Indeed, many of us would rather it wasn’t there at all, because we think the resulting model is inelegant or even ugly, and are trying to think of other cosmological models that do not require  its introduction.

In other words, Dark Energy is an expletive (though not one that’s been deleted).

Incidentally, one of the things Andrew said in his talk – and I agree with him 100% – is that in some sense we already know enough about dark energy from observations that we know we don’t understand it at all from a theoretical point of view. Bigger and better surveys, such as Euclid, producing more and more data will characterize its properties with greater statistical accuracy, but they won’t on their own solve the Dark Energy puzzle. For that we need better theoretical understanding.

My own view is that the problem of the vacuum energy is of the same character as the ultraviolet catastrophe that ushered in the era of quantum physics: a big problem that needs a big solution. What I mean by that is that it’s not something that can be resolved by tinkering with the existing theoretical framework. Something much more radical is needed.

A Flight Through the Universe

Posted in The Universe and Stuff with tags , , on August 15, 2012 by telescoper

Today I’m taking a flight back from Copenhagen to London, a flight through a very small part of the Universe, so it seems apt to put it in perspective by posting this nice video produced on behalf of the the Sloan Digital Sky Survey. I’ve even had the nerve to copy the blurb:

This animated flight through the universe was made by Miguel Aragon of Johns Hopkins University with Mark Subbarao of the Adler Planetarium and Alex Szalay of Johns Hopkins. There are close to 400,000 galaxies in the animation, with images of the actual galaxies in these positions (or in some cases their near cousins in type) derived from the Sloan Digital Sky Survey (SDSS) Data Release 7. Vast as this slice of the universe seems, its most distant reach is to redshift 0.1, corresponding to roughly 1.3 billion light years from Earth. SDSS Data Release 9 from the Baryon Oscillation Spectroscopic Survey (BOSS), led by Berkeley Lab scientists, includes spectroscopic data for well over half a million galaxies at redshifts up to 0.8 – roughly 7 billion light years distant – and over a hundred thousand quasars to redshift 3.0 and beyond.

Click here for more information about BOSS and the latest data release.

News from the BOSS

Posted in The Universe and Stuff with tags , , , on April 1, 2012 by telescoper

No April Fool’s from me today I’m afraid!

New results from the Baryon Oscillation Spectroscopic Survey (known to its friends as BOSS) were one of the highlights of the National Astronomy Meeting last week (which I wasn’t at) and they’ve received quite a lot of press attention over the past few days. Rather than repeat what’s been said I thought I’d reblog this lengthy piece, which gives a lot of detail and is also written by an insider!

we are all in the gutter

I wrote the following post yesterday, but I fell asleep before I could do anything with it. It’s about the first set of results from the Baryon Oscillation Spectroscopic Survey (BOSS), part of Sloan Digital Sky Survey-III project, which we announced to the science community and to the press yesterday. How this whole project was picked up by the press in a way I hadn’t anticipated is the matter for another post. What really matters is the science, and the science – if you don’t mind my exceedingly biased opinion – is just excellent.


I’m now making my way back home from this year’s National Astronomy Meeting (NAM) 2012 in Manchester. I love NAM. It’s always a chance to see old friends and listen to good science, to catch up on gossip and long-promised pints. This year, I did almost none of these things. The reason is that one…

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