Archive for February 2, 2017

The Dipole Repeller

Posted in The Universe and Stuff with tags , , , , , , on February 2, 2017 by telescoper

An interesting bit of local cosmology news has been hitting the headlines over the last few days. The story relates to a paper by Yehuda Hoffman et al. published in Nature Astronomy on 30th January. The abstract reads:

Our Local Group of galaxies is moving with respect to the cosmic microwave background (CMB) with a velocity 1 of VCMB = 631 ± 20 km s−1and participates in a bulk flow that extends out to distances of ~20,000 km s−1 or more 2,3,4 . There has been an implicit assumption that overabundances of galaxies induce the Local Group motion 5,6,7 . Yet underdense regions push as much as overdensities attract 8 , but they are deficient in light and consequently difficult to chart. It was suggested a decade ago that an underdensity in the northern hemisphere roughly 15,000 km s−1 away contributes significantly to the observed flow 9 . We show here that repulsion from an underdensity is important and that the dominant influences causing the observed flow are a single attractor — associated with the Shapley concentration — and a single previously unidentified repeller, which contribute roughly equally to the CMB dipole. The bulk flow is closely anti-aligned with the repeller out to 16,000 ± 4,500 km s−1. This ‘dipole repeller’ is predicted to be associated with a void in the distribution of galaxies.

The effect of this “void in the distribution of galaxies” has been described in rather lurid terms as “Milky Way being pushed through space by cosmic dead zone” in a Guardian piece on this research.

If you’re confused by this into thinking that some sort of anti-gravity is at play, then it isn’t really anything so exotic. If the Universe were completely homogeneous and isotropic – as our simplest models assume – then it would be expanding at the same rate in all directions.  This would be a pure “Hubble flow“, with galaxies appearing to recede from an observer with a speed proportional to their distance:

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But the Universe isn’t exactly smooth. As well as the galaxies themselves, there are clusters, filaments and sheets of galaxies and a corresponding collection of void regions, together forming a huge and complex “cosmic web” of large-scale structure. This distorts the Hubble flow by inducing peculiar motions (i.e. departures from the pure expansion). A part of the Universe which is denser than average (e.g. a cluster or supercluster) expands less  quickly than average, a part which is less dense (i.e. a void) expands more quickly than average. Relative to the global expansion rate, clusters represent a “pull” and voids represent a “push”. That’s really all there is to it.

The difficult part about this kind of study is measuring a sufficient number of peculiar motions of galaxies around our own to make a detailed map of what’s going on in the local velocity field. That’s particularly hard for galaxies near the plane of the Milky Way disk as they tend to be obscured by dust. Nevertheless, after plugging away at this for many years, the authors of the Nature paper have generated some fascinating results. It seems that our Galaxy and other members of the Local Group lie between a dense supercluster (often called the Shapley concentration) and an underdense region, so the peculiar velocity field around us has an approximately dipole structure.

They’ve even made a nice video to show you what’s going on, so I don’t have to explain any further!

 

 

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You have the power to change someone’s life – Time to Talk Day 2017

Posted in Mental Health with tags , , on February 2, 2017 by telescoper

time-to-talk-day

Today, 2nd February 2017, is Time to Talk Day, which means that it’s time for the nation’s biggest conversation about mental health.

One in four adults and one in ten young people will experience a mental health problem every year. Talking about it doesn’t have to be difficult but can make a big difference. It’s easy to have a conversation about mental health, and it could change someone’s life (including yours). You don’t have to be an expert to help: sometimes just asking  how they are is all you need to do to help someone who’s having a hard time.

Here’s a little video about Time to Talk Day:

For more information on how to get involved see here.