Tag: Big Bang

‘Surface of last screaming’

This has nothing to do with anything in the news. I was reading up about the Big Bang for a blog post when I came across this lucid explanation – so good it’s worth sharing for that reason alone – for the surface of last scattering, the site of an important event in the history of the universe. A lot happens by this moment, even if it happens only 379,000 year after the bang, and it’s easy to get lost in the details. But as the excerpt below shows, coming at it from the PoV of phase transitions considerably simplifies the picture (assuming of course that you’re comfortable with phase transitions).

To visualise how this effect arises, imagine that you are in a large field filled with people screaming. You are screaming too. At some time t = 0 everyone stops screaming simultaneously. What will you hear? After 1 second you will still be able to hear the distant screaming of people more than 330 metres away (the speed of sound in air, v, is about 330 m/s). After 3 seconds you will be able to hear distant screams from people more than 1 kilometre away (even though those distant people stopped screaming when you did). At any time t, assuming a suitably heightened sense of hearing, you will hear some faint screams, but the closest and loudest will be coming from people a distance v*t away. This distance defines the ‘surface of last screaming’ and this surface is receding from you at the speed of sound. …

When something is hot and cools down it can undergo a phase transition. For example, hot steam cools down to become water, and when cooled further it becomes ice. The Universe went through similar phase transitions as it expanded and cooled. One such phase transition … produced the last scattering surface. When the Universe was cool enough to allow the electrons and protons to fall together, they ‘recombined’ to form neutral hydrogen. […] photons do not interact with neutral hydrogen, so they were free to travel through the Universe without being scattered. They decoupled from matter. The opaque Universe then became transparent.

Imagine you are living 15 billion years ago. You would be surrounded by a very hot opaque plasma of electrons and protons. The Universe is expanding and cooling. When the Universe cools down below a critical temperature, the fog clears instantaneously everywhere. But you would not be able to see that it has cleared everywhere because, as you look into the far distance, you would be seeing into the opaque past of distant parts of the Universe. As the Universe continues to expand and cool you would be able to see farther, but you would always see the bright opaque fog in the distance, in the past. That bright fog is the surface of last scattering. It is the boundary between a transparent and an opaque universe and you can still see it today, 15 billion years later.

Disastrous hype

This is one of the worst press releases accompanying a study I’ve seen:

The headline and the body appear to have nothing to do with the study itself, which explores the creative properties of an explosion with certain attributes. However, the press office of the University of Central Florida has drafted a popular version that claims researchers – who are engineers more than physicists – have “detailed the mechanisms that could cause the [Big Bang] explosion, which is key for the models that scientists use to understand the origin of the universe.” I checked with a physicist, who agreed: “I don’t see how this is relevant to the Big Bang at all. Considering the paper is coming out of the department of mechanical and aerospace engineering, I highly doubt the authors intended for it to be reported on this way.”

Press releases that hype results are often the product of an overzealous university press office working without inputs from the researchers that obtained those results, and this is probably the case here as well. The paper’s abstract and some quotes by one of the researchers, Kareem Ahmed from the University of Central Florida, indicate the study isn’t about the Big Bang but about similarities between “massive thermonuclear explosions in space and small chemical explosions on Earth”. However, the press release’s author slipped in a reference to the Big Bang because, hey, it was an explosion too.

The Big Bang was like no other stellar explosion; its material constituents were vastly different from anything that goes boom today – whether on Earth or in space – and physicists have various ideas about what could have motivated the bang to happen in the first place. The first supernovas are also thought to have occurred a few billion years after the Big Bang. This said, Ahmed was quoted saying something that could have used more clarification in the press release:

We explore these supersonic reactions for propulsion, and as a result of that, we came across this mechanism that looked very interesting. When we started to dig deeper, we realized that this is relatable to something as profound as the origin of the universe.

Err…

Our universe, the poor man’s accelerator

The Hindu
March 25, 2014

On March 17, radio astronomers from the Harvard-Smithsonian Center for Astrophysics, Massachusetts, announced a remarkable discovery. They found evidence of primordial gravitational waves imprinted on the cosmic microwave background (CMB), a field of energy pervading the universe.

A confirmation that these waves exist is the validation of a theory called cosmic inflation. It describes the universe’s behaviour less than one-billionth of a second after it was born in the Big Bang, about 14 billion years ago, when it witnessed a brief but tremendous growth spurt. The residual energy of the Bang is the CMB, and the effect of gravitational waves on it is like the sonorous clang of a bell (the CMB) that was struck powerfully by an effect of cosmic inflation. Thanks to the announcement, now we know the bell was struck.

Detecting these waves is difficult. In fact, astrophysicists used to think this day was many more years into the future. If it has come now, we must be thankful to human ingenuity. There is more work to be done, of course, because the results hold only for a small patch of the sky surveyed, and there is also data due from studies done until 2012 on the CMB. Should any disagreement with the recent findings arise, scientists will have to rework their theories.

Remarkable in other ways

The astronomers from the Harvard-Smithsonian used a telescope called BICEP2, situated at the South Pole, to make their observations of the CMB. In turn, BICEP2’s readings of the CMB imply that when cosmic inflation occurred about 14 billion years ago, it happened at a tremendous amount of energy of 1016 GeV (GeV is a unit of energy used in particle physics). Astrophysicists didn’t think it would be so high.

Even the Large Hadron Collider (LHC), the world’s most powerful particle accelerator, manages a puny 104 GeV. The words of the physicist Yakov Zel’dovich, “The universe is the poor man’s accelerator”— written in the 1970s — prove timeless.

This energy at which inflation has occurred has drawn the attention of physicists studying various issues because here, finally, is a window that allows humankind to naturally study high-energy physics by observing the cosmos. Such a view holds many possibilities, too, from the trivial to the grand.

For example, consider the four naturally occurring fundamental forces: gravitation, strong and weak-nuclear force, and electromagnetic force. Normally, the strong-nuclear, weak-nuclear and electromagnetic forces act at very different energies and distances.

However, as we traverse higher and higher energies, these forces start to behave differently, as they might have in the early universe. This gives physicists probing the fundamental texture of nature an opportunity to explore the forces’ behaviours by studying astronomical data — such as from BICEP2 — instead of relying solely on particle accelerators like the LHC.

In fact, at energies around 1019 GeV, some physicists think gravity might become unified with the non-gravitational forces. However, this isn’t a well-defined goal of science, and doesn’t command as much consensus as it submits to rich veins of speculation. Theories like quantum gravity operate at this level, finding support from frameworks like string theory and loop quantum gravity.

Another perspective on cosmic inflation opens another window. Even though we now know that gravitational waves were sent rippling through the universe by cosmic inflation, we don’t know what caused them. An answer to this question has to come from high-energy physics — a journey that has taken diverse paths over the years.

Consider this: cosmic inflation is an effect associated with quantum field theory, which accommodates the three non-gravitational forces. Gravitational waves are an effect of the theories of relativity, which explain gravity. Because we may now have proof that the two effects are related, we know that quantum mechanics and relativity are also capable of being combined at a fundamental level. This means a theory unifying all the four forces could exist, although that doesn’t mean we’re on the right track.

At present, the Standard Model of particle physics, a paradigm of quantum field theory, is proving to be a mostly valid theory of particle physics, explaining interactions between various fundamental particles. The questions it does not have answers for could be answered by even more comprehensive theories that can use the Standard Model as a springboard to reach for solutions.

Physicists refer to such springboarders as “new physics”— a set of laws and principles capable of answering questions for which “old physics” has no answers; a set of ideas that can make seamless our understanding of nature at different energies.

Supersymmetry

One leading candidate of new physics is a theory called supersymmetry. It is an extension of the Standard Model, especially at higher energies. Finding symptoms of supersymmetry is one of the goals of the LHC, but in over three years of experimentation it has failed. This isn’t the end of the road, however, because supersymmetry holds much promise to solve certain pressing issues in physics which the Standard Model can’t, such as what dark matter is.

Thus, by finding evidence of cosmic inflation at very high energy, radio-astronomers from the Harvard-Smithsonian Center have twanged at one strand of a complex web connecting multiple theories. The help physicists have received from such astronomers is significant and will only mount as we look deeper into our skies.