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Cosmic Variance on bloggingheads.tv

By Mark Trodden | June 20, 2009 11:24 am

As Sean mentioned yesterday, earlier this week the two of us recorded an episode of bloggingheads.tv, which appeared this morning and can be viewed below.

Although Sean is a veteran, it was my first time, and I wasn’t really sure what to expect, and as a result I was a little apprehensive about recording it. However, once we got going it was very enjoyable, and rather quickly it no longer felt so odd to be chatting over the phone while staring at myself on the monitor.

An hour sped by as we discussed the constituents of the universe, the mysteries surrounding baryonic matter, dark matter and cosmic acceleration, and just touched on the anthropic approach to the cosmological constant problem. We’re planning to do another one of these before too long, in which I think we’ll discuss inflation and more esoteric topics such as the early universe, the multiverse and (I strongly expect) the arrow of time.

Hope you enjoy it.

Update: Here’s a link to the bloggingheads.tv page, where a variety of download formats are available.

CATEGORIZED UNDER: Science, Science and the Media
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  • QUASAR

    I think that you two should discuss the fact that our Universe is a universe of decay, based on our current knowledge!

  • Patrick

    Thanks, enjoyed this a lot. Look forward to more.

  • ZachB

    Haha, seesaw mechanism.

  • http://bjkeefe.blogspot.com/ Brendan

    I did enjoy this diavlog, very much, and I hope you and Sean do at least one more of these soon.

  • http://thesciencepundit.blogspot.com/ The Science Pundit

    I really enjoyed that. Unfortunately, I’m just as confused about the deeper cosmological questions now as I was before watching. Maybe one of these days it will all start to make sense. :-/

  • http://telescoper.wordpress.com Peter Coles

    For some reason this reminded me of Thunderbirds.
    FAB!

  • Starwatcher162536

    Hi, here are a few questions I had after watching you two on Bloggingheads.

    [B](.5A)[/B]Is there anyway to prove that we are not in the center of the universe with a linear redshift-distance law as opposed to being in the center with a quadratic redshift-distance law?

    [B](A)[/B]I was under the impression, that since anything outside the Hubble radius is receding at values above c{z>C}, current theory is that the universe is still inflating, albeit much much slower then right after the Big Bang. Is dark energy what is suppose to be driving this inflation?

    [B](B)[/B]If the above is true, then that means that a'(t) and a”(t) are >0, has this been measured? Is this why Hubble’s constant is said to be decreasing? But doesn’t this contradict the inflation model, since if a'(t) is increasing exponentially (like I would expect if the expansion depends on the volume of the universe) the constant should have a steady value, whats going on?

    [B](B.5)[/B]Is there any difference between dark energy and zero point energy? If not, how can dark energy actually produce any effects? I thought the only reason you could subtract out the infinities of ground states is because only differences in energy are measurable. But since it is producing changes, does this mean the zero point energy is interacting with something outside our universe that has a even lower ground state?

    [B](C)[/B]Since the Freidman equations depend on a isotropic universe (k can only have discrete values), what methods do we have to measure the receding velocities of distant objects that do not depend on the redshift?

  • http://blogs.discovermagazine.com/cosmicvariance/sean/ Sean

    Starwatcher– If you believe we live in a special place in the universe, very strange distance/redshift relations are possible. There are some constraints, but as I recall (I may be out of date) they’re not very good.

    Objects sufficiently far away are always moving faster than the speed of light, if the expanding universe is big enough. Dark energy is responsible for accelerated expansion: the apparent velocity of individual galaxies is increasing with time. “Inflation” refers specifically to accelerated expansion in the early universe.

    Yes, we think that a’ and a” are both positive right now. That’s from real measurements. The Hubble parameter is a’/a, which is actually decreasing. although slowly.

    Zero point energy is one possible example of dark energy. It produces effects through its influence on gravity (spacetime curvature). “Only differences in energy are observable” is not true once you include gravity.

    It’s hard to measure velocities without using redshifts. You might look at Ned Wright’s tutorial for more info:

    http://www.astro.ucla.edu/~wright/cosmolog.htm

  • Steve

    Since it’s just talking heads, it would be nice to have an mp3 file of the audio to listen to while away from the computer.

  • http://bjkeefe.blogspot.com/ Brendan

    @Steve:

    If you visit the diavlog page, you can find a link to the MP3 file containing the audio-only portion of the diavlog, right below the video window and list of topics. (You can also download video files (WMV or MP4), for anyone else who might be interested.)

  • Dave

    I don’t usually have the chance to listen to two astrophysicists describing the universe. Sometimes the internets are great.

  • Shantanu

    Sean, I think I may have asked you this. But do you think that Milgrom’s relations could be telling
    us something about the nature of the dark matter (rather than modified gravity) and that it cannot be a
    a vanilla WIMP, but has to be more exotic.? AFAIk,
    the only person who has advocated this pov is Luc Blanchet who argued that dark matter consists of dipolar medium. Or does the bullet cluster rule out his proposed dark matter model?
    Nevertheless from listening to this conversation, it seems you agree that Milgrom’s relations are a fact and have to be explained, right?
    Thanks

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Cosmic Variance

Random samplings from a universe of ideas.

About Mark Trodden

Mark Trodden holds the Fay R. and Eugene L. Langberg Endowed Chair in Physics and is co-director of the Center for Particle Cosmology at the University of Pennsylvania. He is a theoretical physicist working on particle physics and gravity— in particular on the roles they play in the evolution and structure of the universe. When asked for a short phrase to describe his research area, he says he is a particle cosmologist.

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