Archive for the The Universe and Stuff Category

Status of Dark Matter in the Universe [CEA]

Posted in The Universe and Stuff with tags , on January 11, 2017 by telescoper

Courtesy of arXiver, here’s a nice review article if you want to get up to date with the latest ideas and evidence about Dark Matter…

arXiver

http://arxiv.org/abs/1701.01840

Over the past few decades, a consensus picture has emerged in which roughly a quarter of the universe consists of dark matter. I begin with a review of the observational evidence for the existence of dark matter: rotation curves of galaxies, gravitational lensing measurements, hot gas in clusters, galaxy formation, primordial nucleosynthesis and cosmic microwave background observations. Then I discuss a number of anomalous signals in a variety of data sets that may point to discovery, though all of them are controversial. The annual modulation in the DAMA detector and/or the gamma-ray excess seen in the Fermi Gamma Ray Space Telescope from the Galactic Center could be due to WIMPs; a 3.5 keV X-ray line from multiple sources could be due to sterile neutrinos; or the 511 keV line in INTEGRAL data could be due to MeV dark matter. All of these would require further confirmation in other experiments…

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The Path of the 2017 Total Solar Eclipse

Posted in The Universe and Stuff with tags , , , on January 6, 2017 by telescoper

I thought I’d share this nice NASA video showing the path of totality of the solar eclipse which will take place on 21st August 2017. This is he determined by the changing position of the shadow cast on the Earth’s surface by the Moon as the Earth rotates beneath it. As you can see the shadow will cross the United States of America from Oregon in the North West to South Carolina in the South East. It even passes over Kansas City on the way, so this promises to be a phenomenon that very many people will experience and enjoy.

Eclipses are not particularly rare: there are at least two every year, but most of these are partial rather than total and it is less common for totality to be witnessed from highly populated areas.

For much more information about the 2017 total eclipse of the sun, see the NASA page here.

Perihelion and the Seasons

Posted in The Universe and Stuff with tags , , , , , , , on January 4, 2017 by telescoper

Today the Earth is at the point on its orbit at which it is at its closest to the Sun, i.e. at its perihelion. To be precise, this event takes place at 14.18 GMT today 4th January 2017; aphelion (the furthest distance from the Sun) is  at 20.11 GMT on July 3rd 2017. You can find a list of times and dates of perihelion and aphelion for future years  here.

It surprises me how many people think that the existence of the seasons has something to do with the variation of the Earth’s distance from the Sun as it moves in its orbit. The fact that perihelion occurs in the depth of winter should convince anyone living in the Northern hemisphere that this just can’t be the case, as should the fact that it’s summer in the Southern hemisphere while it is winter in the North.

The real reason for the existence of seasons is the tilt of the Earth’s axis of rotation, as explained in a nice little video here. I used to do a little demonstration with a torch (flashlight to American readers) to illustrate this when I taught first-year astrophysics. If you shine a torch horizontally at a piece of card it will illuminate a patch of the card. Keep the torch at the same distance but tilt the card and you will see the illuminated patch increase in size. The torch is radiating the same amount of energy but in the second case that energy is spread over a larger area than in the first. This means that the energy per unit area incident on the card is decreases when the card is tilted. It is that which is responsible for  winter being  colder than summer. In the summer the sun is higher in the sky (on average) than in winter. From this argument you can infer that the winter solstice (which passed on 21st December), not the perihelion, is the relevant astronomical indicator of winter.

That is not to say that the shape of the Earth’s orbit has no effect on temperatures. The eccentricity of the Earth’s orbit is e=0.017. Estimate the percentage difference in the flux of energy arriving at Earth from the Sun at the extremes of its orbit (i.e. at perihelion and aphelion).

I leave it as an exercise for the student that the fractional difference in distance between perihelion and aphelion in an elliptical orbit is 2e. The fractional change in flux received between the two extremes is thus 4e or about 6.8%, which is indeed a significant in the insolation at the Earth’s surface.

Assuming (for the sake of illustration only)  that the Earth behaves like a black body  then the incident flux would go as the fourth power of the temperature, which means that the temperature should change by about 1.7% between perihelion and aphelion. Taking the mean temperature to be about 300 K the difference should be around 5° C.

Ignoring any effects other than insolation this means that summer in the Southern hemisphere (when the Earth is at perihelion) should be significantly warmer than summer in the Northern hemisphere (when the Earth is at aphelion).

Things are not as simple as that, however, because the Earth’s surface possesses a significant North-South asymmetry: there is a much larger fraction of ocean in the Southern hemisphere, for example, which could be responsible for moderating. The climate is a non-linear system that involves circulating air and ocean currents that respond in complicated ways and on different timescales not just to insolation but to many other parameters, including atmospheric composition (especially water vapour).

The perihelion effect on summer temperatures can be readily verified by observations of Mars, which has no oceans and a much thinner, drier atmosphere, as well as a much more eccentric orbit (e=0.0934).

 

P.S. Of course, at perihelion the Earth is not only closest to the sun but also moving with its greatest orbit speed. That no doubt explains why January is often very windy…

P.P.S. That was a joke.

 

Back to Work…

Posted in Biographical, The Universe and Stuff with tags , , on January 3, 2017 by telescoper

Well, the Christmas break is over at Cardiff University and I’m back in the office of the Data Innovation Research Institute. To be honest, it’s rather quiet around here. Most staff seem to be still on holiday. There are a few students around, mainly international ones. This is actually a revision week at Cardiff University in advance of the mid-year examinations which start next week and go on for a fortnight. After that we’ll be back into teaching. I’ll be doing a Masters-level module on The Physics of the Early Universe in the forthcoming term, and I’m very much looking forward to it.

The outcomes of the annual round of consolidated grants administered by the Astronomy Grants Panel of Science and Technology Facilities Council were announced just before Christmas, with success for some and disappointment for others. I only have anecdotal evidence from personal contacts but it seems to have been a tough round, which wouldn’t surprise me because the funding for basic scientific research in the UK has been flat in cash terms for many years now, and is gradually being eroded by inflation. It’s a tough climate but when, in a couple of years, we lose access to all forms of EU funding things will get even tougher…

Anyway, as new grants are announced and old ones terminated, this is a busy time of year for postdocs (who are largely funded by research grants) seeking new positions. I’ve spent most of the day so far writing references for applicants and will return to that task for a couple of hours after lunch. It’s particularly tough on those whose positions lapse at the end of March who only got notice just before Christmas that their existing funding is not going to be renewed. There’s little time in such a position to get a new job sorted, but on the other hand, new grants are starting from 1st April so there are opportunities out there. It’s not easy to respond if you have a family or other commitments, though.

Another thing that happened just before Christmas was that the Data Innovation Research Institute here at Cardiff University announced its first tranche of “seedcorn” grants to foster interdisciplinary research. These grants are quite small in cash terms but it is hoped that at least some of them will help develop substantial projects by bringing together parts of the University that don’t previously collaborate enough. Congratulations to those whose proposals were selected, and commiserations to those who were unsuccessful.

I was pleased that my proposal – together with Professor Nikolai Leonenko of the School of Mathematics – was one of the successful bids. That means that, probably in the spring, we will be organizing a short workshop relating to the analysis and modelling of astrophysical data defined on the sphere, a topic which has interesting mathematical aspects as well as very practical implications for astronomy and cosmology. We’ll be starting to organize that soon, which adds another item to my to-do list, but it should be a fun conference when it happens.

Before you ask: yes, I do work for the Data Innovation Research Institute but because I was an applicant I recused myself from judging the applications in case there was any perception of a conflict of interest. So there.

Most of my work between now and the start of teaching term is going to be devoted to a couple of MSc courses we’re planning to launch this year, but I’ll write more about them – and plug them shamelessly – when they’re all formally announced and ready to go!

And with that I’d better get back to work again.

Exploring Cosmic Origins with CORE: Inflation [CEA]

Posted in The Universe and Stuff on January 3, 2017 by telescoper

More on the capabilities of CORE…

arXiver

http://arxiv.org/abs/1612.08270

We forecast the scientific capabilities of CORE, a proposed CMB space satellite submitted in response to the ESA fifth call for a medium-size mission opportunity, to improve our understanding of cosmic inflation. The CORE mission will map the CMB anisotropies in temperature and polarization in 19 frequency channels spanning the range 60-600 GHz. CORE will have an aggregate noise sensitivity of $1.7 mu$ K$cdot ,$arcmin and an angular resolution of 5′ at 200 GHz. We explore the impact of telescope size and noise sensitivity on the inflation science return by making forecasts for several instrumental configurations. This study assumes that the lower and higher frequency channels suffice to remove foreground contaminations and complements other related studies of component separation and systematic effects, which will be reported in other papers of the series “Exploring Cosmic Origins with CORE.” We forecast the capability to determine key inflationary parameters, to lower the…

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Exploring Cosmic Origins with CORE: Cosmological Parameters [CEA]

Posted in The Universe and Stuff on January 3, 2017 by telescoper

Here’s an arXiver post I missed before Christmas about CORE – an exciting proposed for new CMB mission with a focus on polarization

arXiver

http://arxiv.org/abs/1612.00021

We forecast the main cosmological parameter constraints achievable with the CORE space mission which is dedicated to mapping the polarisation of the Cosmic Microwave Background (CMB). CORE was recently submitted in response to ESA’s fifth call for medium-sized mission proposals (M5). Here we report the results from our pre-submission study of the impact of various instrumental options, in particular the telescope size and sensitivity level, and review the great, transformative potential of the mission as proposed. Specifically, we assess the impact on a broad range of fundamental parameters of our Universe as a function of the expected CMB characteristics, with other papers in the series focusing on controlling astrophysical and instrumental residual systematics. In this paper, we assume that only a few central CORE frequency channels are usable for our purpose, all others being devoted to the cleaning of astrophysical contaminants. On the theoretical side, we assume LCDM as…

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Scientific Breakthrough of the Year 2016

Posted in The Universe and Stuff with tags , , on December 31, 2016 by telescoper

The year 2016 is almost over and there are just few hours left  until a 2017 begins. Looking back over the scientific discoveries of the last 12 months, I expect you think I would choose the discovery of gravitational waves by the LIGO consortium as my “Scientific Breakthrough of the Year”.

Wonderful achievement though that was, I have, after due reflection, decided to award the accolade to something else which has even more profound implications for the human race and its place in the Universe.

So without further ado, I hereby announce that the In The Dark award for Scientific Breakthrough Of The Year 2016 goes to Donald Trump,  for providing us at last with a definitive resolution of the Fermi Paradox.

 

I hope this clarifies the Apocalypse.