Friday, March 22, 2013

Exciting News from the Planck Satellite

We live in a weird universe. It had a beginning. It is enormous, probably infinite, in extent. And yet at least our observable part of it started out in a volume smaller than the head of a pin 13 to 14 billion years ago. In the vast unobservable part of the universe, there may well be an exact copy of you reading this blog. The universe is getting bigger, but the expansion rate started out fast, slowed down, and is now speeding up again. Is that weird enough for you? No? How about all that and lopsided? Read on.

All this would sound crazy if we didn't have a lot of evidence that tells us that this is the universe we inhabit. That evidence includes:
  • The observation that almost all galaxies in the sky are moving away from us, and that the ones that are further away are moving away from us faster. 
  • The fact that we cannot find stars or galaxies that look like they are much older than about 14 billion years old (plus or minus a few billion). 
  • The observation that relatively pristine gas in the universe consists of about 73% hydrogen and 25% helium. This ratio was set in place during the few minutes during cosmic expansion when the universe was cool enough for protons and neutrons to exist, but hot enough and dense enough for them to be continually bumping into each other.
And the evidence includes measurements of the Cosmic Microwave Background (CMB) radiation - radio waves that are all around us. The CMB accounts for a very small fraction of the static you can hear on an FM radio if you tune between channels. These radio waves originated when the universe was only 380,000 years old and was still permeated by a dense fog of electrons that were bumping into each other too often to be bound to atoms.

The big news this week is the release of the first cosmology results from the Planck satellite, which is the latest in series of increasingly sophisticated observatories designed to measure the statistical properties of this radio noise from the early universe. I'm going to comment on a few of interesting results in the 29 papers that were just submitted for publication. I am not an expert on the measurements and I have only just begun reading the papers. I'm aiming here for a set of highlights that are a bit more detailed than the press release, but easier to digest than the papers. There are some nice blog posts at Bad Astronomy and Starts with a Bang as well. If you are an aficionado, feel free to comment if you find that I've botched something, or missed something important!

The Data

The map of the Cosmic Microwave Background from the Planck satellite, 
after removing the foreground sources of radio emission. 
The Planck satellite is a radio telescope that scans the sky repeatedly. By comparing the radio power it receives when pointing in different directions, it can build up a map of the temperature fluctuations in the gas that permeated the universe when it was only 380,000 years old. These temperature fluctuations are tiny -- less than one part in 10,000. The engineering feat of creating devices to measure this is quite amazing. There are also other sources of radio emission in the sky, and it has taken a lot very careful work to measure these sources and account for their effect on the Planck measurements. Once these "foreground" sources are removed, the result is a map of the surface of last scattering. The places where you see red are just a tiny bit hotter than the places where you see blue. These hotter and cooler spots agree remarkably well with the previous best measurements from the Wilkinson Microwave Anisotropy Probe (WMAP) satellite.

A blowup of a small portion of the CMB map. The new map on the right is from Planck. The previous best map from the WMAP satellite is on the left. The Planck measurements agree extremely well, but obviously provide more detail. From the European Space Agency.

You can see by eye that these maps are not uniform. There are fluctuations from pixel to pixel. But these aren't completely random. There are somewhat coherent patches that are bluer and other patches that are redder. The tool that astronomers use to quantify this is the angular power spectrum. The basic concept of measuring the angular power is something like the following. Draw a little circle on the map. Measure the average temperature in that circle. Do that in a bunch of other little circles that are the same size and compare the results. If they are all the same, write down the number zero. If they aren't,  you can quantify the dispersion of the measurements using the standard deviation or some other statistical quantity. Now vary the size of these circles. You will find that the dispersion in these mean temperatures depends on the size of the circle. Plot up these statistical results as a function of the angular separation and you have an angular power spectrum, more or less. (In practice, the fluctuations are measured using Fourier analysis and spherical harmonics). The figure below shows the angular power spectrum from Planck.

The angular power spectrum of temperature fluctuations measured by Planck. Small angular separations are on the right, large angles are on the left. The red points show the Planck measurements. The green curve is the prediction from the standard model of cosmology -- that weird model described at the opening of this blog post. (If you want to understand what causes the wiggles, I highly recommend reading  Wayne Hu's web site and looking at some of his animations.) A couple of amazing things to note. (1) Every point has an error bar. Most of them are tiny. This is  a great measurement. (2) The predictions from the cosmological model go right through the data. The shaded green region at large angular scales shows the uncertainty due to the fact that we can only observe from one vantage point whereas the models predict the fluctuations that would be observed for an ensemble vantage points placed randomly throughout the universe. From the European Space Agency.

What Does It Mean?

The standard (weird) model of cosmology is expressed as a series of mathematical equations, which include physical constants that we know quite well (for example the speed of light), and include other quantities, like the density of matter in the universe, that we don't know as well. These quantities are known as cosmological parameters. There are seventeen parameters involved in the model represented by the green curve in the angular power spectrum plot. Having adjusted the parameters to fit the data, it's then possible to infer other quantities -- like the current rate of expansion of the universe, its age, and the mean density of matter and energy. 

So here are a few highlights of the Planck findings.

The universe is expanding more slowly than we thought.

The present-day expansion rate of the universe is known as the Hubble Constant. The Planck estimate is only about 90% of the expansion rate that we measured a couple years ago with the Hubble Space Telescope. This is a bit surprising given the estimates of the uncertainties from both measurements. We usually learn something when two sets of precision measurements don't quite agree. So it's going to be interesting to try to understand what the source of the discrepancy is. 

The universe has only three flavors of neutrinos.

One of the interesting inferences from the previous, higher, expansion rate was that the model that provided the best fit to all the data required more than three types of neutrinos. Neutrinos are very light subatomic particles that carry no charge and rarely interact with atoms. There about 100 million neutrinos passing through you right this second. The standard model of particle physics has three types of neutrinos. The previous cosmological observations suggested there were four. Because this would have a profound impact of our understanding of the forces of nature, the measurements created a lot of interest in a hypothetical particle called a sterile neutrino, which interacts with normal matter only through gravity. If the sterile neutrino had mass, it could be part of the dark matter that pervades the universe. It would be warm dark matter, and its existence might help resolve some of the problems that we encounter when trying to explain galaxy properties using just cold dark matter. If Planck is right, we will need to look elsewhere for solutions.

The universe is flat.

It's that time of year, so imagine yourself on the surface of a giant basketball, but one that is poorly inflated. If you start walking one direction, you will eventually end up where you started. If you count your steps, you can estimate the size of the basketball. Now imagine someone inflates the basketball. It's still more or less spherical, but it will take more steps to arrive back at your starting point. The number of steps gives you the size. The fact that you ended up where you started, tells you that the basketball is a closed surface. Another thing that tells you this is that two parallel lines will eventually intersect -- like those two lines that meet in the middle of the picture. 

The universe is harder to picture because the "surface" has three dimensions, not just two. If the universe is closed, a beam of light sent out in one direction will eventually (billions of year later) return to you. In the CMB measurements, changing the curvature of the universe changes the position of the peaks in angle on the sky. The Planck observations confirm what previous measurements were telling us, that the universe is spatially flat. Two lines that start out parallel won't intersect, and they won't drift apart. Flatness is one of the consequences of the early, rapid expansion of the universe called inflation. So it is good to see this holding up. 

The universe has a bit less Dark Energy and a bit more Dark Matter than we thought.

We don't know what Dark Matter and Dark Energy are. Dark Matter is probably some kind of subatomic particle that interacts with normal matter primarily through gravity. We think it is there because we can measure its gravitational influence on large scales in the universe. There are many, many observations that are best explained by invoking dark matter. But we still don't know what it is. Dark Energy is even stranger. It has been invoked to explain the observation that the universe has started to expand faster. It is probably has some link to Vacuum Energy in particle physics, but there is no consensus among physicists on how it is related. The best fit to the wiggles in the Planck power spectrum suggests that the universe consists of 68% Dark Energy, 27% Dark Matter and 5% normal matter (what all of the stars and galaxies and people in the universe are made of). The previous best estimate was 72% Dark Energy.

Dusty galaxies can be detected by their mass.

This result is interesting to those of us who study distant galaxies. There is a fluctuating background at far-infrared wavelengths that has nothing to do with the CMB. It probably arises from dusty galaxies, but we don't know much about them, other than that they are very numerous and probably very distant. The dust absorbs the optical and ultraviolet radiation emitted from stars and re-emits it in the infrared. Planck can measure this fluctuating background in its short wavelength channels, and by correlating with the radio maps, can see the statistical effect of gravitational lensing of the CMB by the mass associated with the dusty galaxies. The average dark-matter mass inferred from the measurement is only about 3% of the mass of the Milky Way. I find this a bit surprising because we tend to find that massive galaxies are very dusty and low-mass galaxies are not. On the other hand, the uncertainties of the estimate are quite large. The constraints might get tighter with more analysis.
This is a cool measurement. In addition to making radio maps, Planck can measure far-infrared radiation that comes from dusty galaxies. The maps in the right panel show the fluctuations in the infrared brightness of the sky at wavelengths of 350 and 545 microns due to these dusty galaxies. You can't see the individual galaxies, just their combined effect. The mass of each of these individual galaxies can imprint a gravitational lensing effect on the CMB. It's possible to detect that effect (the diagrams on the left and center) by stacking the CMB maps at the positions of the peaks (left) and troughs (center) of the far-infrared map. By measuring the effect, it's possible to infer the mass and distance of the galaxies responsible for the far-infrared radiation. Paper XVIII.

The Universe is lopsided?

This is the result that might engender the most discussion, because it suggests the universe might be even weirder than we thought. A fundamental assumption of cosmology is that this vast universe looks statistically the same in all directions. There is no preferred direction. There was a hint in the earlier data from the WMAP satellite that there might be a preferred direction. That hint has gotten much stronger with the Planck data, which confirms both the effect and the direction. One interpretation is that our entire galaxy is moving in one direction faster than we think it should if we add up the pull of gravity from all the other galaxies in that direction. But that interpretation doesn't fit all the data, and there are weirder possibilities that involve changing some of our assumptions about the cosmological model.

And one other tidbit that's going to cause problems.

The Planck papers have a lot of co-authors. The the authors are listed alphabetically after the words "Planck Collaboration." The first author is almost always Peter Ade of Cardiff University. It's rare (but not unprecedented) in astronomy to construct the author lists this way, because the tradition has been to recognize the people who have contributed most to a particular paper (even if it is from a giant collaboration like CANDELS) by listing their names near the top of the author list. By bucking tradition, the Planck collaboration is acknowledging  that it often impossible to judge who made the most significant contributions to a particular paper, particularly if it depends on the technical efforts of many, many other people. On the other hand, it is going to be interesting to see who gets invited to the conferences!

Thursday, March 14, 2013

Astronomer of the Month: Tomas Dahlen

Each month we will highlight a member of the CANDELS team by presenting an interview introducing them and what it's like to be an astronomer. This month's Astronomer is Tomas Dahlen.

Tell us a little about yourself!

My name is Tomas Dahlen. I'm an Associate Scientist at Space Telescope Science Institute in Baltimore, MD, where I'm been for about six years since moving from my native Stockholm, Sweden. 

What is your specific area of research? What is your role within the CANDELS team? 

I'm a co-investigator in the CANDELS team. I'm particularly interested in the supernova aspect of the survey, deriving rates of both Type Ia supernovae and core collapse supernovae and figuring out what they can tell us about for example the history of star formation and metal enrichment. I'm also involved in deriving photometric redshifts to the galaxies in the different fields.

What made you want to become an astronomer? At what age did you know you were interested in astronomy? 

Some of my earliest memories in life are associated with the Apollo program. I remember watching the landing on the Moon and playing with those space age toys and listening to "Major Tom". I think I've always kept that fascination for space and astronomy, but didn't for a long time imaging that it was something that I could actually work with. I graduated with a Masters of Science in Physical Engineering from the Royal Institute of Technology in Stockholm, not including any courses in astronomy. However, I did take a course in special relativity which was nice. It wasn't until my early 30s that a took a couple of evening courses in astronomy that I realized that this could actually be something to make into a career. So I took a full year of undergraduate course and thereafter started my PhD studies at the Stockholm Observatory. I completed my PhD in 2002 and went for a first Post-doc at STScI working with the GOODS collaboration. After a second Post-doc, this time in Stockholm, I went back to STScI 2006 and has been there since.

What obstacles have you encountered on your path to becoming an astronomer and how did you overcome them? 

I think that the uncertainty of the job situation after getting a PhD is an obstacle that many have to struggle with. You spend many years on studies and then Post-docs not really knowing if and when you will be able to get a more permanent position down the road. This can be hard, but so far I've been lucky to be able to find work at places I really like.
Who has been your biggest scientific role model and why? 

No one really specific I guess. But after taking that early course in relativity, I've been fascinated and intrigued about the thought processes that must have been going around in Einstein's head.

What is it like to be an astronomer? What is your favorite aspect? 

It is of course very nice to work on something that you really like. And you are at times trying to help answer questions that have yet been answered. That's a nice thought. And it takes you places all around the world when collaborating with other scientists, making your own observations, or attending conferences.  

What motivates you in your research? 

This is related to above, finding out just a small bit of the big picture.  

What is your favorite astronomical facility? (This could include telescopes or super computers, for example) 

Having worked with the remarkable Hubble for a number of years, this has to be my favorite. Of telescopes that I've actually touched, the Nordic Optical Telescope on La Palma has a special place since I spent numerous nights there during my PhD. And of course also the retired 50/60cm diameter, 8 meter focal length, Double Refractor telescope from 1930 at the Observatory in Saltsjobaden in Sweden. I held a series of public shows at this telescope during the winter months in the mid 90's. Boy were people surprised when the dome floor started to move up or down when the telescope changed position and found it hard to believe that it was actually Saturn with its rings they were looking at and not an image someone had put in the telescope.

Where do you see yourself in the future? What are your career aspirations? 

I hope to continue working within the field, including inspiring collaborations such as CANDELS.
If you could have any astronomy related wish, what would it be? 
To find something no one had expected in the next CANDELS supernova search.

What is your favorite, most mind-boggling astronomy fact? 

Just the mere size of space and time is amazing. The light packet from a distant exploding supernova has been traveling for many billions of years when we detect it with Hubble, a few weeks later it is gone.

Is there anything else you would like for the public to know about you or astronomy in general? 

When I am not working on astronomy, I take my telescope outside; but not for star gazing at night, but rather birding any time of the day. I've been a devoted birder my whole life and combining this with the possibilities to travel that the profession as an astronomer gives, provides great opportunities to visit and bird different parts of the world.

Tuesday, March 12, 2013

Observing Comet PanSTARRS

Comet PanSTARRS viewed from Gate's Pass near Tucson, AZ, image credit & copyright: Janine Pforr
In the last post, we gave you some background on comets and highlighted a few of the more suspected-to-be-spectacular comets visible this year. Last night, a few colleagues and I went to a nearby viewpoint, to do some comet PanSTARRS viewing. 

mage credit & copyright: Janine Pforr

We chose Gate's Pass near Tucson, which regularly offers a great view of the sunset and a brilliant night sky when it's clear. We arrived shortly after the sun had set (around 6:30 pm) and the sky was still a beautiful yellow, orange and red. Clearly, other people had the same idea - the viewpoint was filled with spectators equipped with cameras,  tripods, and binoculars, ready to spot the comet. We waited anxiously after reports from colleagues who were not able to see comet PanSTARRS at all the evening before and some who said they only saw it with binoculars. Around 7pm I heard the first people say that they spotted it in their binoculars. After a couple of test pictures and asking others for the exact comet location, we saw it, too! Once we knew where to look, even with the naked eye, we could just about see it. The comet started being visible above the horizon near where the sun had set. With the camera it was a bit easier because of the longer exposures. We were all happy that we could actually see it and now also have picture-proof. We watched the comet set slowly following the sun for about 30 minutes (see video).  

Time lapse video of Comet PanSTARRS setting at Gates Pass in Tucson, Arizona. Image credit and copyright: Janine Pforr. You can find a higher resolution of this video on our Facebook page at:

If you are planning to go out and watch comet PanSTARRS, today is probably your best chance as comet PanSTARRS is expected to get a little bit brighter still. The best time to look is about 40 minutes after the sun has set. However, I do recommend to take binoculars with you, or a small telescope, to get a closer up view! We hope you enjoy the sight and would love to hear about your viewing experiences. Over the next few weeks, the comet will get fainter and further away from the sun. This will make it harder to see with the naked eye, but easier to see with a telescope. Some observatories, such as Kitt Peak, will have special events for viewing the comet.

Tuesday, March 5, 2013

2013 - The Year of Comets

Comets are small solar-system objects. They are often referred to as dirty snowballs because they are believed to mainly consist of ice and dust. But they also contain things like methane, ammonia, carbon dioxide etc.

Illustration of a comet's tails. Image credit: NASA
When comets come close to the sun, the radiation from the sun causes some of the comet material to be released, i.e. the ice is turned into gas and any dust within it is freed. This forms a coma of material around the comet's core; it's a little bit like an atmosphere. Due to the solar wind this material is pushed away from the comet and leaves a visible tail behind. In fact comets can have more than one tail. There is a gas tail that points in the opposite direction of the sun which consists mostly of gas atoms that are ionized by the suns radiation and a tail of dust grains that leans a little bit more towards the comets trajectory (see the illustration). The core of a comet is thought to be a few tens of miles/kilometers in size or smaller, the coma on the other hand can reach a million miles or more; that's about the size of the sun! Some comets can also have very long tails with some of the longest reported tails being as long as about 1 Astronomical Unit; that's the distance between the Earth and the sun!  

The tail and coma are what makes comets easily distinguishable from asteroids. However, every time a comet passes by our Sun it loses some of its material until eventually all the ice has gone and the only remainder might be a piece of rock. 

It is believed that most comets originate from the formation of our solar system. They are left-overs that didn't make it into a planet or moon. They mostly live in the Kuiper Belt and what is called the Oort Cloud. The Oort Cloud is a described as a sort of spherical area far out around our solar system that harbors a vast number of icy objects. Occasionally some of these collide or encounter other massive objects (such as the gas planets) that disturb their regular orbit. And sometimes the new path the comet adopts will lead through the inner solar system. When these objects come close to the Sun we observe them as comets. Some comets come by on a regular basis, such as the famous Halley's comet. They found a stable new orbit that can take the comet anything from a few years to more than 100,000 years to complete once. Comets with periods shorter than about 200 years are called short-period comets and are believed to come from the Kuiper Belt, those with longer periods on the other hand are called long-period comets coming most likely from the Oort Cloud. Rarer are those comets that only pass by once and are kicked out of the solar system forever. These comets are called hyperbolic comets, named after the shape of their trajectory.

Path of Comet PanSTARRS, Image credit: NASA
In the past we have had the pleasure to see many great comets in the night sky, the greatest ones even with the naked eye. This year, 2013, promises to be another great year for bright comets. There will be 2 very bright comets, one moderately bright one (Comet C/2012 F6 Lemmon) and one regular visitor (Comet 2P/Encke). Let me tell you here about the 2 brightest ones. The first one predicted to be relatively bright is comet PanSTARRS (official designation C/2011 L4 PanSTARRS) this month. It's got its name from the PanSTARRS survey which discovered this comet. Scientists predict that the brightness of this comet is going to be around as bright as the stars in the big dipper but brightness predictions are difficult. While being visible from the Southern Hemisphere already, comet PanSTARRS will be visible from the Northern Hemisphere starting March 7th (this Thursday), just above the horizon after sunset.
Comet McNaught in 2007, Image credit: ESA/NASA
Another great show will be put on at the end of November by comet C/2012 S1 ISON, which some say is expected to be brighter than the full moon!! This is indeed a rare occasion. Two Russian amateur astronomers discovered Comet ISON while observing for the International Scientific Optical Network (ISON) which gave the comet its name. Calculations of the comet's orbit revealed that it will pass very close to the sun (less than 1 million miles distance). Comets that come this close to the sun are called sungrazers. In the case of surviving this close pass to the sun, the view of this comet should be spectacular, possibly similar to that of Comet C/2006 P1 McNaught in 2007 (see picture). I will sure keep an eye out this year and try to spot one of these passers-by!

Friday, March 1, 2013

The week of the HST deadline

Hubble Space Telescope, Image credit: NASA
You might have been wondering why the blog has been so quiet this week. Well, the deadline for another round of proposals using the Hubble Space Telescope (HST) is this Friday, March 1st. Many CANDELS team members are caught up in frantically writing and finishing observation proposals. An observational proposal requires a lot of work. Not only does one have to present a scientifically interesting idea, the so-called science justification, for the use of an expensive facility like the HST, the proposal writers also need to put together a technical justification. The latter includes a more detailed observation strategy that outlines number and length of the proposed observations and which available instruments will be used. Since often many astronomers build a team (just like CANDELS although usually a lot smaller) to propose for their idea, everybody is involved in putting the proposal together and constantly communicates with each other right up until the deadline to make their proposal as strong as possible. For HST several types of proposals can be submitted. The first type is the so-called archival and theoretical proposal, which is mainly a proposal for funding to support one's research. As the name indicates, astronomers base their research idea upon already existing observations in the HST archive or theoretical work that they need support for. The second type is proposing to carry out new observations with HST.

Like all telescopes, observing time with HST (and funding) is highly competitive and many more astronomers will submit proposals than there is time available (or money to spend). However, we won't know which proposals were accepted before the end of May. Once an observing proposal is accepted it has to successfully pass Phase II. This includes working with an assigned program coordinator at the Space Telescope Science Institute in Baltimore, that runs HST, to put together a plan for the observations which then can be scheduled on HST. This observational plan has to be submitted by the end of June, so only about a month after one has been notified that the proposal was accepted. However, a proposal is only fully accepted once all the technical details for the observation plan have been successfully worked out, it has been assured that the same (or very similar) observations are not carried out more than once (e.g. different proposers want to observe the same piece of sky in the same filter bands and depths or an observation exists already in the archive) and it has been checked that all observational requirements are technically possible and feasible for HST. When all these hoops have been jumped through, HST can take the data and astronomers can analyze and interpret it and then publish their results.