cosmology – Scienceandsf -A Blog Published by Robert A. Lawler

Here we go Again. A Recent Paper by a Group of Cosmologists raises doubts about the very Existence of Dark Energy.

We’ve all heard the old saying ‘Two steps forward, one step back’. Well, when it comes to Cosmology, the study of the Universe as a whole, it seems like we take a step forward, another sideways, close your eyes and spin, take two steps etc, etc, you get the idea. The Universe is so large, the measurements so difficult to make, the theories so complex that progress in cosmology has always been slow with many wrong turns. So hang on folks, today’s post is going to be a little trip with Alice into wonderland.

Today the best model we have for the basic nature of the Universe is that is consists of billions of Galaxies like our Milky Way. That the Universe is expanding, all those Galaxies are moving away from each other, and that the expansion is not being slowed by the gravity of the Galaxies. In fact the expansion is accelerating. This basic model is outlined in the image below.

It was Carl Hubble, back in the 1920s and 30s who discovered that the Universe was made of Galaxies and that it was expanding. The acceleration of the Universal expansion was discovered in the 1990s by two groups of astronomers led by Saul Perlmutter and Adam Riess.

The cause of this acceleration was completely unknown and quickly given the name ‘Dark Energy’, although cosmologists prefer the name ‘Vacuum Pressure’. Today we know almost nothing about ‘Dark Energy’ and it ranks as one of the greatest mysteries in all of science.

Now a recent paper published by Lawrence H. Dam, Asta Heinesen and David L. Wiltshire of the University of Canterbury in New Zealand may be about to throw the whole science of cosmology into a state of confusion. According to Professor Dam and his colleagues there is no such thing as Dark Energy, it simply doesn’t exist. Cosmologists only think there’s Dark Energy because they’re trying to fit their measurements to an incorrect mathematical model of the Universe.

To understand what Professors Dam, Heinesen and Wiltshire are saying we need to talk a little bit about the mathematical ideas we use to describe the Universe and of course we start with Albert Einstein. When Einstein published his General Theory of Relativity, also known as his Theory of Gravity, it was quickly realized that since it was gravity that held the Universe together then Einstein’s Gravity theory was the best way in which to study the Universe. The full Einstein equation for gravity is shown below, it’s the lambda (L) symbol that relates to Dark Energy.

A trio of physicists named Alexander Friedman, Howard Robertson, and Arthur Walker used Einstein’s theory to develop an exact set of equations for a Universe where matter was spread smoothly (homogenous) and the same in every direction (isotropic). A mathematician named Georges Lemaitre later expanded the FRW model to include the expansion of the Universe thereby creating the ‘Big Bang Theory’, although technically it is referred to as the FLRW model.

Now remember the two assumptions of the FLRW model, that the matter in the Universe is smoothly distributed with no preferred direction, i.e. it is homogenous and isotropic. At first glance however the Universe sure doesn’t look smooth, it’s got the Galaxies, clusters of stars with a whole lot of empty space between them. However, the idea was that when you considered the whole Universe with tens of billions of Galaxies they would all spread out evenly.

Except that they don’t. Another important astronomy project of the last twenty years has been the Sloan Digital Sky Survey (SDSS), an ambitious attempt to map the positions of nearly a million Galaxies and what the Sloan team has discovered is that the Universe actually looks more like Swiss cheese or soap bubbles with regions that are quite dense surrounding immense empty voids. The image below shows a sample of the results of the SDSS and clearly illustrates the ‘lumpiness’ of the Universe.

Results of Sloan Digital Sky Survey (Credit: SDSS)

So the basic assumptions of the FLRW model aren’t quiet right and Professors Dam, Heinesen and Wiltshire say that a new mathematical model, which they call the Timescape model, must be used instead. It’s in this mathematical model that the measurements made by Perlmutter and Riess fit without the need for anything like Dark Energy.

Now there’s a long way to go before the Timescape model is generally accepted, if it ever is. Chances are that this theory will not stand the test of close examination and Dark Energy will continue to be a mystery that needs to be solved. You never know though, every time we look further into the Universe it just seems to get stranger and stranger.

I realize that this post was rather long and heavy and dealt with some strange and difficult topics. However I hope that it wasn’t too abstract. The intersection between math and measurement is central to the advance of science and after all, we are taking about the basic structure of the Universe as a whole!

Was Einstein Wrong??? Is the Speed of Light not Constant???

Over the past week there have been a series of news articles reporting that two physicists, Niayesh Afshordi at the University of Waterloo in Canada along with Joao Magueijo at the Imperial College of London have proposed that Einstein may have been wrong. The Speed of Light may not be constant, right after the Big Bang it may have been a lot faster.

Do you want the short answer or the long answer. For the short answer read the next 3 paragraphs, for the long answer keep going. If you want to read the news report use the link below.

http://www.csmonitor.com/Science/2016/1128/Einstein-s-speed-of-light-theory-tested-Did-he-get-it-wrong

What Afshordi and Magueijo were looking for is a solution to the problem in cosmology of just how the early universe was in such good thermal equilibrium as is evidenced by the Cosmic Microwave Background, CMB see picture below. For different objects, at different initial temperatures to come into thermal equilibrium requires some kind of contact between those different objects. In this case we are taking about the entire early Universe which is flying apart at the speed of light and that ain’t good contact.

Cosmic Microwave Background from Plank Satellite

What Afshordi and Magueijo have proposed is that, in the Early Universe the speed of light was far greater than it is now allowing greater thermal contact by the process of radiation. Remember there are three ways for heat to flow: conduction, convection and radiation, well Afshordi and Magueijo’s model would make radiation a much more efficient process thereby eliminating the thermal contact problem.

The point to remember here is that this is all mathematics at present, no one has measured a different value for the speed of light. Afshordi and Magueijo do make a prediction of the scalar fluctuations in the CMB as an experimental check but at the moment this is all just a model.

Also, we’ve been here before. The problem of thermal equilibrium in the CMB goes back to the 1970s when Alan Guth of MIT proposed cosmic inflation as the solution. The idea of inflation was that, right after the big bang itself, and we’re taking pico-seconds here, a huge amount of energy was dropped into the universe causing it to expand faster than the speed of light so that a small section of the universe that was in thermal equilibrium became the entire universe that we see. For thirty years after Guth published his model inflation was a standard part of cosmology, I learned it, but no one has been able to figure out where all that energy came from so inflation is no longer quite so highly regarded.

To me however, this new idea of Afshordi and Magueijo is just kind of the opposite of inflation. Instead of having a small part of the universe after the big bang expand faster than the speed of light they increase the speed of light, in a sense making the early universe smaller. And they now have the problem of describing what made the speed of light so different, and what makes it so constant now? Kind of the opposite of Guth’s problem of where all that energy came from. I wish Afshordi and Magueijo luck but as I said, we’ve been here before.

Now I get to give my opinion. To me the reason the early universe was in thermal equilibrium right after the big bang was that it was in thermal equilibrium before the big bang. That’s right I’m one of those big crunch guys, that is I think that about 15 billion years ago, before the big bang, the universe was collapsing at the speed of light. Eventually the universe collapsed as much as it could and then rebounded, that rebound is what we call the big bang. A universe that is collapsing is coming into greater contact and therefore will achieve thermal equilibrium before the rebound, giving it thermal equilibrium after the rebound.

Anyway, that’s what I think. I know this has been a bit of a long post but I hope you enjoyed it. Let me know what you think.

Does Dark Energy really Exist?

For the past twenty years the greatest mystery in all of Science has been the Nature of Dark Energy, a unknown force that is accelerating the expansion of the Universe and whose energy makes up more than three quarters of everything there is. Now a new study by Cosmologists J.T.Nielsen, A. Guffanti and S. Sarkar has called into question the very existence of Dark Energy.

To understand what is going on we have to go back to 1929 when astronomer Edwin Hubble discovered that the galaxies he was studying were all receding from our milky way and that the further a galaxy was the faster it was receding. This is Hubble’s law for the expansion of the Universe and the rate of expansion is called Hubble’s constant.

Almost immediately after Hubble’s announcement physicists and astronomers began to theorize what could have caused this expansion and so they developed the Big Bang Theory which was finally confirmed by A. Penzias and R. Wilson in 1965. But if the Big Bang Theory was true then the gravitational attraction of the galaxies should be slowing the rate of expansion, Hubble’s constant should not be truly constant. Cosmologists also theorized that there could be two basic solutions. One, the gravitational attraction was strong enough that eventually the expansion would come to a stop and the entire Universe would enter a Big Crunch phase. The other solution was that the expansion was so great that the Universe had achieved escape velocity and would expand forever.

It was to discover which of these two alternative Universes was true that two teams of astronomers, one led by S. Perlmutter and the other by A. Riess and B. Schmidt used Type Ia supernova to try to measure the deceleration of Hubble’s constant. In 1998 these two teams independently announced their findings that the expansion of the Universe was in fact accelerating, a discovery that shocked the world of science and led to Perlmutter, Riess and Schmidt being awarded the 2011 Nobel Prize in Physics.

The search was now on for the cause of this acceleration, an unknown force that was quickly named Dark Energy although cosmologists prefer to call it Dark Pressure. Literally hundreds of theoretical papers have been written over the past two decades speculating on the nature of Dark Energy but little more observational detail was provided by astronomers.

Now a new study, using data from more than ten times as many Type Ia supernovas as was available to Perlmutter, Riess and Schmidt has called into question the very existence of Dark Energy, asserting that the discovery was if fact only a statistical fluctuation. This new study by astronomers J.T. Nielsen, A. Guffanti and S. Sarkar uses data from 740 Type Ia supernovas and concludes that “we find, rather surprisingly, that the data are still quite consistent with a constant rate of expansion.”

This result is something of a shocker, have cosmologists spent the last 18 years chasing a phantom? Personally I’ll wait and see. There is other indirect evidence for Dark Energy in the Cosmic Microwave Background so Dark Energy isn’t gone just yet. In fact the new study found that the evidence for Dark Energy is at the three sigma level but scientists prefer five sigma so we are talking statistical weights of evidence.

Still this is yet another example of just how difficult it is to get real, precise details on the nature of our Universe. We have learned a great deal but that knowledge has required great effort, and just as often great patience. I’ll keep you informed.

I’m going to try a little experiment of my own and try to insert the actual article by Nielsen et al into this post. Enjoy.

srep35596

Here we go again. The Universe just got ten times bigger!

A team of astronomers led by Christopher Conselice at the University of Nottingham in the UK have been studying the deep field images coming from the Hubble Space Telescope and concluded that the Universe contains ten times as many galaxies as was previously thought.

The previous census performed by the Sloan Digital Sky Survey, SDSS (link below) concluded that the observable Universe contained between 100-200 Billion galaxies.

http://classic.sdss.org/

This new census realized that the density of galaxies in the early Universe was far greater than it is now and that many of these early galaxies were too faint to be seen in the data used by the SDSS. On the basis of their data from the Hubble they have realized that the Universe contains on the order of one Trillion galaxies.

As exciting as this discovery is it’s really nothing new. Every time we study the Universe with new, more powerful, more precise instruments the Universe grows ever larger. Sometimes the expansion is linear as with this census by Dr. Conselice and his team, sometimes it is exponential as when Carl Hubble himself discovered that the smudgy nebula he studied were actual galaxies separate from our Milky Way. By the way, the Greek word galaxy just means Milky Way. From Galileo to Dr. Conselice we have learned that the Universe is more than we can ever imagine.

I have seen this phenomenon of expansion happen three or four times now in my life and I expect to see it happen at least one more time.The James Webb space telescope is expected to be launched sometime in 2018 and with this new window to infinity I have no doubt that the Universe will grow once again.

To read further about the new census follow in link below.

http://www.spacetelescope.org/news/heic1620/

Does the Universe have a Preferred Direction

One of the basic assumptions that Astronomers and Cosmologists start with as the try to understand this Universe we live in is that on the very largest scales it’s the same in every direction. That is, when we look deep into space at all the galaxies and clusters of galaxies, and voids between galaxies, the Universe looks pretty much the same in whatever direction we look. This property is know as isotropic.

This assumption is very basic to our understanding of reality. When I taught physics I always tried to impress on my students how, when you’re trying to solve a problem, you can put your origin anywhere you want and point your x, y and z axis in whatever direction want in order to make the problem easier to solve. So this idea is not only fundamental, it is also very useful.

Assumptions have to be tested however, and a group of Cosmologists at University College in London have used the data obtained by the Planck satellite’s observations of the Cosmic Microwave Background (CMB) to see if they could find any evidence that our Universe had a preferred direction or even if it had a spin. Now the CMB is radiation left over from the very earliest time after the big bang, photons of light that have whizzed through space for over thirteen billion years without interacting with any other particles giving us a baby picture of our Universe. (See the Image at the top)

The Cosmologists looked at the CMB data for any signs for elongations or spiral patterns that would indicate a preferred direction or an axis of rotation and they calculate that there is only a 1 in 121,000 chance that there is any anisotropic (non-isotropic) behavior in the CMB.

So it appears that all of our theories that are based on an isotropic Universe are still good, for now. In another decade or so another group of scientists will think of another way of testing this assumption with even greater precision and that’s as it should be. As human being we have to make assumptions, but we have to test them again and again.

For those who are interested, you can read more about the work of the University College of London cosmologists here:

http://phys.org/news/2016-09-scientists-universe.html