Robert Scherrer explains Dark Energy

April 23, 2015 § Leave a comment

Explainer: the mysterious dark energy that speeds the universe’s rate of expansion

Robert Scherrer, Vanderbilt University

The nature of dark energy is one of the most important unsolved problems in all of science. But what, exactly, is dark energy, and why do we even believe that it exists?

What goes up must come down… right?
Ball image via www.shutterstock.com.

Step back a minute and consider a more familiar experience: what happens when you toss a ball straight up into the air? It gradually slows down as gravity tugs on it, finally stopping in mid-air and falling back to the ground. Of course, if you threw the ball hard enough (about 25,000 miles per hour) it would actually escape from the Earth entirely and shoot into space, never to return. But even in that case, gravity would continue to pull feebly on the ball, slowing its speed as it escaped the clutches of the Earth.

But now imagine something completely different. Suppose that you tossed a ball into the air, and instead of being attracted back to the ground, the ball was repelled by the Earth and blasted faster and faster into the sky. This would be an astonishing event, but it’s exactly what astronomers have observed happening to the entire universe!

This illustration shows abstracted ‘slices’ of space at different points in time as the universe expands.
Ævar Arnfjörð Bjarmason, CC BY-SA

Scientists have known for almost a century that the universe is expanding, with all of the galaxies flying apart from each other. And until recently, scientists believed that there were only two possible options for the universe in the future. It could expand forever (like the ball that you tossed upward at 25,000 miles an hour), but with the expansion slowing down as gravity pulled all of the galaxies toward each other. Or gravity might win out in the end and bring the expansion of the universe to a halt, finally collapsing it back down in a “big crunch,” just like your ball plunging back to the ground.

So imagine scientists’ surprise when two different teams of astronomers discovered, back in 1998, that neither of these behaviors was correct. These astronomers were measuring how fast the universe was expanding when it was much younger than today. But how could they do this without building a time machine?

Luckily, a telescope is a time machine. When you look up at the stars at night, you aren’t seeing what they look like today – you’re seeing light that left the stars a long time ago – often many hundreds of years. By looking at distant supernovae, which are tremendously bright exploding stars, astronomers can look back hundreds of millions of years. They can then measure the expansion rate back then by comparing the distance to these far-off supernovae with the speed at which they are flying away from us. And by comparing how fast the universe was expanding hundreds of millions of years ago to its rate of expansion today, these astronomers discovered that the expansion is actually speeding up instead of slowing down as everyone had expected.

What pushes galaxies like these in the Hubble deep field apart?
NASA and A. Riess (STScI), CC BY

Instead of pulling the galaxies in the universe together, gravity seems to be driving them apart. But how can gravity be repulsive, when our everyday experience shows that it’s attractive? Einstein’s theory of gravity in fact predicts that gravity can repel as well as attract, but only under very special circumstances.

Repulsive gravity requires a new form of energy, dubbed “dark energy,” with very weird properties. Unlike ordinary matter, dark energy has negative pressure, and it’s this negative pressure that makes gravity repulsive. (For ordinary matter, gravity is always attractive). Dark energy appears to be smoothly smeared out through the entire universe, and it interacts with ordinary matter only through the action of gravity, making it nearly impossible to test in the laboratory.

Scientists used to think that the expansion of the universe was described by the yellow, green, or blue curves. But surprise, it’s actually the red curve instead.

The simplest form of dark energy goes by two different names: a cosmological constant or vacuum energy. Vacuum energy has another strange property. Imagine a box that expands as the universe expands. The amount of matter in the box stays the same as the box expands, but the volume of the box goes up, so the density of matter in the box goes down. In fact, the density of everything goes down as the universe expands. Except for vacuum energy – its density stays exactly the same. (Yes, that’s as bizarre as it sounds. It’s like stretching a string of taffy and discovering that it never gets any thinner).

Astronomers continue to probe the skies, looking for finer details that can build on what we suspect about dark energy.
Reidar Hahn, CC BY

Since dark energy can’t be isolated or probed in the laboratory, how can we hope to understand exactly what it’s made of? Different theories for dark energy predict small differences in the way that the expansion of the universe changes with time, so our best hope of probing dark energy seems to come from ever more accurate measurements of the acceleration of the universe, building on that first discovery 17 years ago. Different groups of scientists are currently undertaking a wide range of these measurements. For example, the Dark Energy Survey is mapping out the distribution of galaxies in the universe to help resolve this puzzle.

Could Einstein’s theory need work?
Sophie Delar

There is one other possibility: maybe scientists have been barking up the wrong tree. Maybe there is no dark energy, and our measurements actually mean that Einstein’s theory of gravity is wrong and needs to be fixed. This would be a daunting undertaking, since Einstein’s theory works exceptionally well when we test it in the solar system. (Let’s face it, Einstein really knew what he was doing). So far, no one has produced a convincing improvement on Einstein’s theory that predicts the correct expansion for the universe and yet agrees with Einstein’s theory inside the solar system. I’ll leave that as a homework problem for the reader.

The Conversation

This article was originally published on The Conversation.
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Zen and the Art of Genetic Manipulation

September 25, 2012 § 2 Comments

All parents want the best for their children. They want them to be safe, happy, and healthy. What if you were able to ensure all of these things for your child through genetic manipulation? The scary part about altering one’s genes, as explored in Gattaca, is not in the prevention of future disease and illness. Because of racially motivated political movements, eugenics has been historically tied to cruel notions about the superiority of certain ethnic groups. This fear of the misappropriation of genetic alteration is reflected often in science fiction, in films like Gattaca and short stories such as Nancy Kress’s Beggars in Spain.

As a parent, contemplating the birth of your child, a malleable, fragile being that you are responsible for shaping, would you elect to give your child the gene for altruism? Often people seem inherently selfish – we make decisions based on sudden self-involved impulse, without considering the larger ramifications of our actions on others. It is easy for us, the few who were lucky enough to be born into a relatively affluent and secure society, to neglect and ignore the needs of the larger community. What if you had the ability to give your child the gift of giving?

When a rabbi wrote to Albert Einstein, asking for advice on what to tell his daughter about God after the death of her sister, Einstein wrote the following in response:

“A human being is a part of the whole, called by us “Universe,” a part limited in time and space. He experiences himself, his thoughts and feelings as something separated from the rest, a kind of optical delusion of his consciousness. This delusion is a kind of prison for us, restricting us to our personal desires and to affection for a few persons nearest to us. Our task must be to free ourselves from this prison by widening our circle of compassion to embrace all living creatures and the whole of nature in its beauty.

American society is based on the foundation of selfishness. Capitalism is a competition – it functions on the basis of individuals fighting to advance their own personal desires. Communism in its purest form is unselfish and altruistic; it is the imposition of human selfishness and greed that corrupts the system and leads to its failure. If it were possible to free ourselves from the fetters of self-obsession, maybe we could elevate ourselves as the human species and work together to eliminate social injustices.

However, if it was me faced with the decision of altering my own baby’s basic makeup and forcing this altruism into them, I would probably balk. On an evading-the-point-of-the-question level, the gene would probably just expose them to abuse by other children who had no such physiological compunctions to avoid selfishness. On another level, Einstein concludes his statements by writing that nobody is perfectly able to eliminate self-interest, but that it is “the striving for such achievement is in itself a part of the liberation and a foundation for inner security.” I think that this kind of genetic manipulation is too much like playing God – attempting to alter basic human nature. Maybe some things have to be left to chance. Maybe we need to trust in the basic good intentions of people, and aim for improvement.

I’m Relatively Uplifted

September 14, 2012 § Leave a comment

Blog 3: I’ll be honest. Before this reading assignment, my view of wormholes and time machines was probably similar to that of the majority of people. It made for decent science fiction, and not much else. Upon my reading of Kip Thorne’s discussion of “Sagan questions” and “exotic matter” I will admit to becoming slightly uplifted. To me, the idea that we have yet to discover the greatest secret the universe has to offer is enthralling. It absolutely fascinates me that the universe has hidden great mysteries virtually under our noses. And here we are, just hoping, praying even, to discover them.

As I read further into the chapter, the brilliance of the scientists at the forefront of quantum mechanics and such fields becomes readily apparent. It isn’t their knowledge that amazes; anyone can amass knowledge. It is their imagination. Simply sitting in a room and spinning off revolutionary ideas astounds me. Sure it takes years of education to even be able to comprehend such ideas; however, the act of generating such ideas is something that cannot be taught.

Now, you might be asking, Pranav why did this make you feel uplifted? The notion that there are scientists who devote their valuable days thinking about wormholes and time machines indicates that there is hope. Hope for time machines. Hope for wormholes. As a nerd, nothing pleases me more than the stuff of what was once purely science fiction, becoming reality: a transition from the story book to the science journal.

Yes, we are still nowhere near close to proving the existence of such phenomena let alone creating one (as Sagan suggested in his book) but the simple act of pondering and ruminating about such things increases ones imagination, and wasn’t it Einstein himself who realized the importance of imagination? I guess all this ruminating begs the question: what would we do with a time machine if one was conjured? Isn’t the answer obvious? Completely mess with the 4-D fabric of space-time and create paradoxes on a universal scale. Duh.

As mere humans, we cannot even comprehend the idea that is time. Perhaps it is wise such insignificant creatures as us leave it as is: untouched. Maybe someday we will have the know-how to “mess” with time, but that day is not today. Or tomorrow for that matter. If anything, we need to learn from the past and plan for a future. Not switch the two around.

-Pranav Santapuram

A Scientist’s and a Sociologist’s Impressions of Science Fiction

September 2, 2011 § Leave a comment

I called the scientist and began by asking what science fiction, if any, he had read growing up. He recalled that he had read and watched some science fiction, mentioning Asimov and Captain Video, but that he sometimes became frustrated by the inaccuracies of the science portrayed by the genre. During the conversation, he mentioned that he felt more moved by his mother’s descriptions of Einstein’s theories than by science fiction. He felt that the factual basis of real science made it, in many ways, more exciting than science fiction. Now, his opinion is not meant as an affront to science fiction but as a suggestion that real science find its way into popular culture. For instance, how often does one search PubMed to brush up on recent medical advances?

The sociologist began by telling me he had also not read much science fiction growing up. He was far too engrossed in reading history. He completed The History of the Decline and Fall of the Roman Empire and A History of the English-Speaking Peoples at a rather young age. He, like the scientist, had his curiosity satiated by learning about the facts of the real world.

I do not believe that science fiction inherently has a responsibility to be realistic and accurate, and I do not think the scientist or sociologist expects this from the genre either. The fictional nature of the science is itself responsible for some of the greatest benefits of the genre. The fictional aspect allows the reader to explore places and scenarios inaccessible in the real world. Science fiction also offers opinions on the challenges that might be faced by our society as it advances technologically. Nevertheless, I think it is important that science fiction not displace an individual’s fascination in his own world. If science fiction is the only means by which a person fulfills his scientific curiosity, he will miss out on the exciting science of the world that he inhabits. Just to reiterate, this is not meant to diminish the value of science fiction.  Rather, it is meant to suggest that the fascination elicited by the scientific elements of science fiction can, and should, be sought after in the science of the real world as well.

Erdos0

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