Well the weather again has been alternating between almost letting the nighttime clouds go away and trying to rain and just being overcast, so it has been mostly a time of spending the nights inside reading some of the stacks of magazines that have arrived. So first some comments about the recent OCA general meeting and recommendation for a new series of lectures on an introduction to cosmology.
First up, this month's OCA general meeting we heard Daniel Limonadi, JPL, talk about "Robotic Exploration of Mars - Highlights and Future Directions." This was a very interesting discussion where we heard about past failures and successes and the next series of robotic explorers, including Insight, which was just launched. Daniel was a very positive and enthusiastic speaker.
|Daniel Limonadi, JPL talks about robotic exploration of Mars at OCA general meeting|
We also heard from OCA Jim Benet in his "What's Up" presentation that the month of June will be a very good month for planetary observations. So if you want to get out a find all the planets (I guess we might not include Pluto in this list), now is the time to break out the scopes and go to it! Thanks for the heads up, Jim!
Next, I got a post or email or tweet or something like that from Sean Carroll that he has just released a great five lecture series on an introduction to cosmology. He gave this series of lectures at CERN in 2005 to a bunch of particle physicists and the lectures are still relevant today. Besides, Sean is such a dynamic and clear lecturer and he does a great job of explaining stuff. In fact, for me, this series is one of the best introductions to cosmology that I have gone through. He goes quick but at least now I can go back over my cosmology textbook and have a much better understanding of what is going on. So, check out the free lectures at his blog site:
Last post, we mentioned that the 3rd release of GAIA astrometric data was just announced and that this new release was expected to transform astronomy. The new release has data on 1.7 billion stars while the previous astrometric mission, Hipparcos, had just managed to map 100,000 stars in close proximity to the sun. Now, check out the range covered by this new release in the chart below that shows the projected range as if we were able to see it from outside of our galaxy. In our previous post of May 6, 2018, we explored how measurement of stars near the Earth have been used to measure and extend the cosmic distance ladders used to determine distances beyond the Milky Way.
|Gaia extends stellar distance measurements 100x over Hipparcos (Source: Davide Castelvecchi, Nature 557, 3 May 2018)|
Another article in Nature provided some more details on the search for the very first stars that turned on in our galaxy, a topic we previously discussed in the April 19, 2018 post. It turns out that the first stars leave an imprint on the very low frequency end of the Cosmic Microwave Background (CMB). Remember that the approach used is to look for the red-shifted 21cm radiation from neutral hydrogen atoms. The 21cm radiation, or if you like to think in terms of frequency, which is 1420 MHz, is red-shifted by the expansion of the universe to be in the 15-200MHz range. This means that the signal is detectable with small radio antennas as long as the background, mostly manmade signals here on Earth, do not interfere too much. Check out some of the antennas set up in the Owens Valley as part of the LEDA array.
|Many observatories look for the signal from the very first stars (Source: D. Castelvecchi, Nature 557, 3 May 2018)|
It turns out that after 2 years of searching and analyzing their results to rule out other extraneous noise sources or systematic errors in their system, a team lead by J. Bowman, was able to use their antenna array in the quiet Australian outback, to find the elusive signal. Now the call is out to other radio observatories to confirm the discovery.
The figure below taken from the referenced Nature article shows the red-shifted signal showing up right around 78 MHz. This signal if it indeed is from the 1420MHz hydrogen spin flip transition has been red-shifted by 1 + z = 1420 / 78 = 18.2. So the hydrogen atoms emitted their characteristic spin flip signal back when the universe was only 1 / 18.2 times as large.
|Original redshifted 21cm signal detected by EDGES in Australia (Source: Bowman, et al, Nature 555, 67-70, (2018)|
The other neat thing that can be learned by this observation is about how long it took for all of the hydrogen atoms to be ionized by these early stars. Look again at the attenuation curve vs. frequency and note that the curve starts to dip at about 68 MHz, and doesn't return to the background level until frequency of about 90 MHz.
So to my student eye, the edge of the dip with the lowest red shift, i.e. closer to zero, occurred later in time from the big bang. This suggests that the first stars turned on at a red shift corresponding to about 68 MHz and then when the universe had expanded to a red shift corresponding to 90 MHz, the spin flip signal goes away, which would seem to be correlated with all of the surrounding neutral hydrogen getting ionized, which would result in no spin flipping neutral hydrogen left. The article says these two events occurred about 150 million years after the big bang and ended about 100 million years later. We will have to keep reading the news stories to see if this interpretation bears out.
Astronomers also predict that if this red-shifted signal could be detected at even lower frequencies of say 15 MHz, this signal would come from the end of the so called dark ages, which represents the time the first star in the early universe turned on. The 15-200 MHz frequency band has a lot of manmade noise and astronomers don't expect to be able to see the 15 MHz signal, unless they are able to build a receiving antenna on the far side of the moon.
Theorists who looked at this data find the dip to be deeper than expected and have begun to theorize that one solution to the excessive dip is if the dark matter is made up of dark matter particles that are lighter than currently being searched for. This might be the reason that the search for dark matter particles keeps coming up empty.
Science Squad Gravity Guy, Ken, forwarded a paper describing a group of particle physicists who theorize that dark matter might be made up of much lighter particles, called axions, which could explain the nature of dark matter and also explain why it has not be found as a particle yet. The motivation for paper was not so much to write a new paper just one of hundreds describing what dark matter might be made of, but their approach was to also solve the 80 odd year old mystery of why the solar corona is so much hotter than the solar surface. They suggest ways to test this new theory and hope that it also solves the old mystery. Hmm, we have seen a lot of papers like this so we will just have to wait and see. Thanks for that, Ken!
Anyway, let's take at look at that paper, not so much for its theoretical axion dark matter particle, but its description of the solar corona heating problem, which is neat in of itself. The figure below shows the solar corona temperature being about 100 times hotter than the solar surface. The left panel shows how the solar temperature experiences an abrupt shift and increase in temperature as one moves away from the surface towards the corona. The panel on the right shows how the coronal temperature departs from the standard blackbody curve, in the ultraviolet wavelengths as shown in the dashed lines.
So how does the corona getting hotter than the sun? Many previous studies, beginning I guess with Parker in about 1988, started looking at nanoflares as the way to transport energy from the solar surface to the corona. Nanoflares are smaller versions of the large prominences that release tremendous amounts of energy. It seems that not everyone is convinced, in fact at the recent CSULB physics colloquium we heard that magnetic fields, in addition to nanoflares, are being now looked at as a means of transferring energy to the corona. Anyway, it is a neat topic and we should keep following new findings in this area.
|The solar corona is almost 100x hotter than the solar surface (Source: Raza, et al, 1805.01897v1)|
All of this news from radio astronomy generated more interest in travelling to see some of these radio astronomy observatories, especially the NRAO in Green Bank, WV, which hosts the SARA amateur radio astronomers conference on June 11-13. Unfortunately, our travel schedule is so full of other activity that again, for the third year in a row, I can't make it there. Darn! Well, anyway, we decided that we could make a quick trip to tour the Very Large Array (see below) in New Mexico. So, we are packing our bags for that quick trip and tour and will report on our findings next time. My observing plan, in addition to scoping out and sampling the local margaritas and sopapillas, is to get up at 4:00AM and take a look and photo of the Milky Way, which should be quite visible in those dark skies.
|One configuration of radio antennas at the Very Large Array (VLA) (Source: www.nrao.edu)|
Ok, that is all the astrophysics for this week. I had considered going to the OCA star party, but again the weather forecast just looked too cloudy to pack everything up. Check out the image below taken at about 8:10PM and the clouds are just too heavy for any observing, although up at the OCA site it might have been a little bit better. I guess we just have to keep reading rather than observing!
|Ok, no astronomical observing through these clouds (Source: Palmia Observatory)|