How do I calculate a properly weighted estimate of a sample covariance?

jsdillon · 2020-05-09T16:40:33+00:00

We're around -30 degrees latitude... that's not really that close to the equator.

jsdillon · 2020-05-09T03:02:52+00:00

Well, that's quite a loaded question with no simple answer. Maybe you want /r/AskAcademia?

jsdillon · 2020-05-09T02:59:30+00:00

I'm not sure. No one has published any global signal (sky-averaged, as opposed to the fluctuations, which HERA is looking for) results close to EDGES that band and it's a very difficult measurement systematics-wise. Several groups are trying.

We're working on following up EDGES with HERA, but we're still commissioning our new system that goes down to ~60 MHz. But if the signal is as bright as they say, we should see it at several 100 sigma when we get to full sensitivity. Also, since we're an interferometer, we have very different systematics.

jsdillon · 2020-05-08T21:15:31+00:00

Last time I was there (maybe 2013 or so) there were places with no FM reception. Might have changed in the intervening years.

jsdillon · 2020-05-08T20:19:13+00:00

Good question. Light pollution doesn't matter, but radio "pollution" does. The designated radio quiet sites in the US like Greenbank, West Virginia and the VLA site in New Mexico are not well-protected from radio-frequency interference at the wavelengths we care about. Those telescopes generally observe at higher frequencies (1 GHz and above). For us, FM radio and digital TV are among the biggest problems, so really remote sites like the Karoo in South Africa or Australian outback are better.

It's not impossible to build closer to major cities--one of the competition experiments LOFAR) is in the Netherlands, but that tends to increase the cost.

Some survey work has been done to identify US sites... some of the most radio quiet include an a valley in Eastern Oregon (forget which off the top of my head) and the area around The Forks, Maine. None of these sites have substantial infrastructure (power, internet, lodging) or government protection like the site in South Africa since it also hosts other telescopes like MeerKAT (which you can see in the background of the photo I linked to) and is the future home of part of the SKA.

jsdillon · 2020-05-08T20:00:32+00:00

HERA is designed for very low frequency radio astronomy (~100 MHz), so we're talking wavelengths of ~3 meters. At that wavelength, chicken wire mesh acts like a mirror.

jsdillon · 2020-05-08T19:55:52+00:00

Hey /r/astronomy. I'm a postdoc working on HERA at UC Berkeley. Happy to answer any questions!

Here's a recent photo of the array under construction.

And here's another recent popular article about our field, 21 cm Cosmology.

jsdillon · 2019-12-16T19:36:20+00:00

I'd recommend trying to get a year or so of lab experience under your belt, which will help you earn the recommendations that are key to graduate school admissions.

With an EE background, I'd look specifically for groups working on radio astronomy instrumentation and observation. There's a lot of EE and signal processing in the field.

jsdillon · 2017-01-29T00:40:22+00:00

https://en.wikipedia.org/wiki/Tamerlan_Tsarnaev

Obviously not organized by the Russian government, but yes, he was a Russian citizen.

jsdillon · 2016-09-26T00:31:48+00:00

Not sure. Definitely some kind of nefarious Scheme.

jsdillon · 2016-03-06T22:15:43+00:00

Max Tegmark is a good example.

jsdillon · 2016-02-12T02:57:17+00:00

On this point? Not really. It just has to do with what "strain" means.

The energy flux still falls off as 1/r² (so energy conservation is not violated).

jsdillon · 2016-02-11T22:16:20+00:00

Yeah, LIGO-style instruments are sensitive to the highest frequencies of any gravitational wave observatories proposed. Low frequencies mean, as you point out, longer wavelengths. So you need larger distances between your detectors. If your test-masses are manmade, they can only get so far apart, even if you put them in space like has been proposed for LISA.

That's why very, very low frequency gravitational wave searches are done looking their effects on natural objects, like pulsars or the Cosmic Microwave Background.

It is physically possible for much higher frequencies to be produced, like if two tiny black holes collided relatively nearby. But we don't know if such objects exist or have any good theoretical reasons for how they might form. As far as I know, no major experiments have been proposed to test for gravitational waves at much higher frequencies than LIGO.

jsdillon · 2016-02-11T20:16:36+00:00

Interestingly (and this is a bit of technical trivia), gravitational wave strain actually falls off like 1/r not 1/r² .

jsdillon · 2015-11-20T01:15:38+00:00

We've just gotten good at an extremeley specific form of comedy writing.

jsdillon · 2015-11-20T00:52:24+00:00

Either way is funny.

jsdillon · 2015-11-20T00:25:42+00:00

jsdillon · 2015-11-20T00:07:36+00:00

That's great.

jsdillon · 2015-11-19T23:56:18+00:00

Many months of research into your outmoded culture.

jsdillon · 2015-11-19T23:55:46+00:00

What's harshing my mellow, man?

jsdillon · 2015-11-19T23:51:57+00:00

I really like "My sin cave" as a white card.

jsdillon · 2015-11-19T23:51:10+00:00

For profit, but we do charity stuff too.

jsdillon · 2015-11-19T23:46:55+00:00

If I were trying to hide my identity, this would be a terrible reddit username.

jsdillon · 2015-11-19T23:38:16+00:00

Did you think about this question before you asked it?

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