Archive for October 31, 2016

Hallowe’en Again…

Posted in Biographical, Music with tags , , , on October 31, 2016 by telescoper

It’s Hallowe’en again, and although I feel I should concoct something appropriate, I really don’t have time. I’ve decided therefore to recycle a couple of items I’ve previously posted on this lamentable occasion.

We never had Halloween when I was a kid. I mean it existed. People mentioned it. There were programmes on the telly. But we never celebrated it. At least not in my house, when I was a kid. It just wasn’t thought of as a big occasion. Or, worse, it was “American” (meaning that it was tacky, synthetic and commercialized).

So there were no Halloween parties, no costumes, no horror masks, no pumpkins and definitely no trick-or-treat when I was a lad.

Having never done trick-or-treat myself as a child, I never really had any clue what it was about until relatively recently. I’d always assumed “Trick or Treat?” was a rhetorical question or merely a greeting like “How do you do?”.

In fact my first direct experience of this peculiar custom  didn’t happen until I was in my mid-thirties and had moved to a suburban house in Beeston, just outside Nottingham. I was sitting at home one October 31st, watching the TV and – probably, though I can’t remember for sure – drinking a glass of wine, when the front door bell rang. I didn’t really want to, but I got up and answered it.

When I opened the door, I saw in front of me two small girls in witches’ costumes. Behind them, near my front gate, was an adult guardian, presumably a parent, keeping a watchful eye on them.

“Trick or Treat?” the two girls shouted. Trying my best to get into the spirit but not knowing what I was actually supposed to do, I answered “Great! I’d like a treat please”.

They stared at me as if I was mad, turned round and retreated towards their minder who was clearly making a mental note to avoid this house in future. Off they went and I, embarrassed at being exposed as a social inadequate, retired to my house in shame.

Ever since then I’ve tried to ensure that I never again have to endure such Halloween horrors. Every October 31st, when nightfall comes, I switch off the TV, radio and lights and sit soundlessly in the dark so the trick-or-treaters think there’s nobody home.

That way I can be sure I won’t be made to feel uncomfortable.

Anyway, despite my own  reservations about Hallowe’en, I’ve decided to resurrect the following little video which seems to be appropriate for the occasion. It’s made of bits of old horror B-movies but the music – by Bobby “Boris” Pickett and the Crypt-kickers is actually the second single I ever bought, way back in 1973…

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A Cosmic Microwave Background Dipole Puzzle

Posted in Cute Problems, The Universe and Stuff with tags , , , , , on October 31, 2016 by telescoper

The following is tangentially related to a discussion I had during a PhD examination last week, and I thought it might be worth sharing here to stimulate some thought among people interested in cosmology.

First here’s a picture of the temperature fluctuations in the cosmic microwave background from Planck (just because it’s so pretty).

planck_cmb

The analysis of these fluctuations yields a huge amount of information about the universe, including its matter content and spatial geometry as well as the form of primordial fluctuations that gave rise to galaxies and large-scale structure. The variations in temperature that you see in this image are small – about one-part in a hundred thousand – and they show that the universe appears to be close to isotropic (at least around us).

I’ll blog later on (assuming I find time) on the latest constraints on this subject, but for the moment I’ll just point out something that has to be removed from the above map to make it look isotropic, and that is the Cosmic Microwave Background Dipole. Here is a picture (which I got from here):

dipole_map

This signal – called a dipole because it corresponds to a simple 180 degree variation across the sky – is about a hundred times larger than the “intrinsic” fluctuations which occur on smaller angular scales and are seen in the first map. According to the standard cosmological framework this dipole is caused by our peculiar motion through the frame in which microwave background photons are distributed homogeneously and isotropically. Had we no peculiar motion then we would be “at rest” with respect to this CMB reference frame so there would be no such dipole. In the standard cosmological framework this “peculiar motion” of ours is generated by the gravitational effect of local structures and is thus a manifestation of the fact that our universe is not homogeneous on small scales; by “small” I mean on the scales of a hundred Megaparsecs or so. Anyway, if you’re interested in goings-on in the very early universe or its properties on extremely large scales the dipole is thus of no interest and, being so large, it is quite easy to subtract. That’s why it isn’t there in maps such as the Planck map shown above. If it had been left in it would swamp the other variations.

Anyway, the interpretation of the CMB dipole in terms of our peculiar motion through the CMB frame leads to a simple connection between the pattern shown in the second figure and the velocity of the observational frame: it’s a Doppler Effect. We are moving towards the upper right of the figure (in which direction photons are blueshifted, so the CMB looks a bit hotter in that direction) and away from the bottom left (whence the CMB photons are redshifted so the CMB appears a bit cooler). The amplitude of the dipole implies that the Solar System is moving with a velocity of around 370 km/s with respect to the CMB frame.

Now 370 km/s is quite fast, but it’s much smaller than the speed of light – it’s only about 0.12%, in fact – which means that one can treat this is basically a non-relativistic Doppler Effect. That means that it’s all quite straightforward to understand with elementary physics. In the limit that v/c<<1 the Doppler Effect only produces a dipole pattern of the type we see in the Figure above, and the amplitude of the dipole is ΔT/T~v/c because all terms of higher order in v/c are negligibly smallFurthermore in this case the dipole is simply superimposed on the primordial fluctuations but otherwise does not affect them.

My question to the reader, i.e. you,  is the following. Suppose we weren’t travelling at a sedate 370 km/s through the CMB frame but instead enter the world of science fiction and take a trip on a spacecraft that can travel close to the speed of light. What would this do to the CMB? Would we still just see a dipole, or would we see additional (relativistic) effects? If there are other effects, what would they do to the pattern of “intrinsic” fluctuations?

Comments and answers through the box below, please!