Joseph Smidt

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  • October 31, 2011
  • 12:35 PM

Could The First Stars In The Universe Really Be Annihilating Dark Matter?

by Joseph Smidt in The Eternal Universe

Stars today burn bright through the interactions of elements like hydrogen and helium.  But was that the same story for the first stars in the universe, or was their light dominated by the annihilation of dark matter?  Recently,  Ili et al. took up the challenge of finding out what astronomers should be looking for if this is the cas.

Some background:  If dark matter really is a weakly interacting particle, then shortly after the big bang, when the universe was very hot and dense, dark matter particles should have been rapidly annihilating.  But what if, after gravitational collapse, the original dark matter halos (think giant globs of dark matter being pulled together by gravity) became so dense that it re-triggered the annihilation of dark matter?  If this happened, the light from the first stars would not be mainly from elements like hydrogen and helium interacting, but instead be from dark matter annihilations themselves.

Well how could we detect such a thing?  Well start by looking at the plot below:

Here we see the predicted "star formation rate" versus redshift predicted by their numerical simulations.  What this shows is that the peak star formation of these "dark stars", as they are called, happened at a redshift of around ~12, or ~13 billion years ago, just after the big bang.  (Remember, the higher the redshift the farther back in time.)

When an object emits a lot of light a long time ago, two things happen:

The light becomes redshifted due to the expansion of the universe so that what once was visible light becomes infrared.
They lose a lot of signal at certain wavelengths due to the Lyman alpha forest.  (See plot below).

The plot above shows what happens when you have the combination of 1 and 2, and is the predicted "SED" for these objects meaning, this is how bright the objects are supposed to be at each wavelength.  As you can see, one one hand, since these objects are at high redshift, the light they emit at the ultra-violet and visible wavelengths has been redshifted to the infrared.     Second, because of the Lyman alpha-forest, you see a drop of below certain wavelengths of light.  (If you don't understand all this it's fine, just know there are predictions for how bright these objects should be at each wavelength so we can now go look for something with that pattern.)

Unfortunately, we can stare at these objects at every wavelength.  Instead, we have to use powerful telescopes and stare at them at the specific wavelengths of those telescopes and hope for the best.  For example, this is a prediction of what Hubble could see in the 1.25 and 1.6 micrometer wavelength bands it has:

The solid red and blue curves are the predictions of the "magnitudes" of these objects in the 1.6 (red) and 1.25 (blue) micrometer bands of Hubble.  (Magnitude is a measure of brightness and the smaller the magnitude the brighter the object... not a typo, astronomers do everything backwards...)  So if you could find an object at redshift 10 with Hubble with a magnitude of ~28 in the 1.6 micrometer band and ~30 in the 1.25 micrometer band, that could be a good hint.

The next step would be to rule out such an object being a normal star.  To do this you use spectroscopy:

 Differentiating first galaxies at z>10 from [dark stars] would be possible with spectroscopy: the [dark stars] (which are too cool produce
significant nebular emission) will have only absorption lines while the galaxies are likely
to produce emission lines as well. .Of particular interest would be the HeII emission
lines at λ ∼ 1.6µ as well as Hα lines which would be signatures of early galaxies rather
than [dark stars].
So there you go: find a high redshift object with the right drop-off in brightness in the right wavelength bands with the right spectroscopy properties and you make have a star, not powered by the interactions of hydrogen and helium like stars today, but may in fact be light from dark matter annihilation similar to the theoretical dark matter annihilation that may have been given off just after the big bang.

Cosmin Ilie, Katherine Freese, Monica Valluri, Ilian T. Iliev, & Paul Shapiro (2011). Observing Dark Stars with JWST e-print arXiv: 1110.6202v1

... Read more »

Cosmin Ilie, Katherine Freese, Monica Valluri, Ilian T. Iliev, & Paul Shapiro. (2011) Observing Dark Stars with JWST. e-print. arXiv: 1110.6202v1

  • February 9, 2011
  • 01:03 PM

Current Cosmology From Supernova Data.

by Joseph Smidt in The Eternal Universe

Ariel Goobar and Bruno Leibundgu have recently submitted an article to Annual Review of Nuclear and Particle Science summing up our current understanding of physics from the current set of supernova data. We have accrued quite a lot of supernova data over the years and so it is interesting to take a look at how much we have learned. I will not report everything but will post a few interesting

... Read more »

Ariel Goobar, & Bruno Leibundgut. (2011) Supernova cosmology: legacy and future. To Appear In Annual Review of Nuclear and Particle Science. arXiv: 1102.1431v1

  • January 28, 2011
  • 12:12 PM

Dark Matter Reconstruction From Radio Experiments.

by Joseph Smidt in The Eternal Universe

As photons move through the universe they get gravitationally lensed as the pass by large clumps of matter. (As shown in the image above.) Dark matter, being the dominant form of matter, lenses these photons more than anything.  Therefore, by studying the lensing properties of incoming photons, in principle we can reconstruct what the profiles of the dark matter doing that lensing.

Now, put (

... Read more »

Michael L. Brown, & Richard A. Battye. (2011) Mapping the dark matter with polarized radio surveys. E-Print. arXiv: 1101.5157v1

  • January 25, 2011
  • 01:00 PM

What If Dark Energy Were A Phantom Energy?

by Joseph Smidt in The Eternal Universe

Before we get too far ahead of ourselves, let's remember that dark energy being a cosmological constant fits the data very well and has for years. That said, experimental constraints allow for dark energy actually being an exotic form of phantom energy. (So for the time being we have to allow for the possibility and work out the details.) This was recently done by Dabrowski and Denkiewicz.


... Read more »

Mariusz P. Dabrowski, & Tomasz Denkiewicz. (2009) Exotic-singularity-driven dark energy. AIP Conference Proceedings. arXiv: 0910.0023v1

  • January 19, 2011
  • 12:14 PM

Evidence Against The Universe Being Fine Tuned For Life.

by Joseph Smidt in The Eternal Universe

Many people will tell you that the universe appears fine tuned for life.  Don Page has decided to address this issue scientifically by calculating the best value for the cosmological constant needed to support life in the universe and then comparing it to our own.  His conclusion is that the cosmological constant is actually an example that our universe is not fine tuned for life.


... Read more »

Don N. Page. (2011) Evidence Against Fine Tuning for Life. E-Print. arXiv: 1101.2444v1

  • January 12, 2011
  • 12:31 PM

First Planck Results: The Sunyaev-Zeldovich Effect.

by Joseph Smidt in The Eternal Universe

There's been many bloggers writing about the first Planck results presented here at AAS and in Europe but I would like to write a little more than has been written on the Sunyeav-Zeldovich results as I think they are impressive.  Impressive both in terms of the science we get as well as well as this particular example shows how precise CMB experiments have become.  I will focus on the results

... Read more »

The Planck Collaboration. (2011) Planck Early Results: The all-sky Early Sunyaev-Zeldovich cluster sample. Submitted to A. arXiv: 1101.2024v1

  • January 11, 2011
  • 03:40 PM

The Statistical Mechanics of Money

by Joseph Smidt in The Eternal Universe

Yesterday I listened to a talk by Victor Yakovenko of the University of Maryland about the physics of money and it was quite interesting. I think that after this talk I am finally beginning to understand economics while at the same time I suspect that most economists don't.

In his talk he said that back in 2000 he published a paper on how to apply statistical mechanics to free market economics.... Read more »

XI, N., DING, N., & WANG, Y. (2005) How required reserve ratio affects distribution and velocity of money☆. Physica A: Statistical Mechanics and its Applications, 357(3-4), 543-555. DOI: 10.1016/j.physa.2005.04.014  

  • January 5, 2011
  • 01:42 PM

How Physics Changes With F(R) Gravity.

by Joseph Smidt in The Eternal Universe

Einstein's general relativity rules the roost when it comes to gravity, but soon modifications to standard GR may detected on universal scales with new cosmological data. Fortunately, a realistic modified version of gravity, known as f(R) gravity, makes practical predictions that may be verified in the coming decades.

A recent paper by Motohashi, Starobinsky, and Yokoyama gives a good synopsis

... Read more »

Hayato Motohashi, Alexei A. Starobinsky, & Jun'ichi Yokoyama. (2011) f(R) Gravity and its Cosmological Implications. to be published. arXiv: 1101.0716v1

  • December 30, 2010
  • 12:11 PM

The Effects Of Special Relativity On Planetary Orbits.

by Joseph Smidt in The Eternal Universe

General relativity affects the orbits of planets in ways Newtonian gravity cannot account for. Interestingly, Lemmon and Mondragon explore if special relativity can account for the same behavior predicted by general relativity.   They find that qualitatively it can, but quantitatively it comes up a little short and so the full general relativistic treatment is still needed.

First a reminder: 

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Tyler J. Lemmon, & Antonio R. Mondragon. (2010) First-Order Special Relativistic Corrections to Kepler's Orbits. Submitted to American Journal of Physics. arXiv: 1012.5438v1

  • September 28, 2010
  • 01:23 PM

What Happens When A White Dwarf Collides With A Neutron Star?

by Joseph Smidt in The Eternal Universe

                             Image via WikipediaPaschalidis et al. recently simulated what will happen when a white dwarf collides with a neutron star in a head on collision incorporating the effects of general relativity.

In each case I will list the mass of the white dwarf and neutron star in solar masses, (meaning the mass of these objects after dividing my the mass of the sun) the ratio of

... Read more »

Vasileios Paschalidis, Zachariah Etienne, Yuk Tung Liu, & Stuart L. Shapiro. (2010) Head-on collisions of binary white dwarf--neutron stars: Simulations in full general relativity. Submitted to PRD. arXiv: 1009.4932v1

  • September 27, 2010
  • 12:15 PM

Distinguishing Our Universe From Other Similar Universes In The Multiverse.

by Joseph Smidt in The Eternal Universe

Srednicki and Hartle have raised an interesting concern recently about a limitation on the predictive power of multiverse theories. They observe that in multiverse theories, exact snapshots of our universe happen several times in different places. So if we want to have a physical theory that describes our universe, the one we live in, then the question arises: how can we tell which one it is from

... Read more »

Srednicki, M., & Hartle, J. (2010) Science in a very large universe. Physical Review D, 81(12). DOI: 10.1103/PhysRevD.81.123524  

  • September 22, 2010
  • 01:24 PM

How The Twin Paradox Of Relativity Changes In An Expanding Universe.

by Joseph Smidt in The Eternal Universe

I'm sure most of you have heard of the twin paradox "in which a twin makes a journey into space in a high-speed rocket and returns home to find he has aged less than his identical twin who stayed on Earth."  This paradox has been worked out for special relativity in Minkowski spacetime.  Recently, Boblest et al. worked out the details using general relativity for an expanding universe. (de Sitter

... Read more »

Sebastian Boblest, Thomas Müller, & Günter Wunner. (2010) Twin Paradox in de Sitter Spacetime. E-Print. arXiv: 1009.3427v1

  • September 21, 2010
  • 12:07 PM

How To Possibly Detect Graviton Mass With Gravity Waves/Pulsars.

by Joseph Smidt in The Eternal Universe

Gravitons are the particles that mediate the force of gravity in the analogous way that photons are responsible for the electro-magnetic field.  And like photons, gravitons are thought to be massless.  In fact, assuming general relativity is correct, the mass of the graviton has an upper bound of 7x10-32 eV which is really small. (See bold text at bottom.)  However, for alternative gravity

... Read more »

Kejia Lee, Fredrick A. Jenet, Richard H. Price, Norbert Wex, & Michael Kramer. (2010) Detecting massive gravitons using pulsar timing arrays. Accepted by ApJ. arXiv: 1008.2561v2

  • September 17, 2010
  • 12:49 PM

Cosmology Can Possibly Solve the Neutrino Hierarchy Problem.

by Joseph Smidt in The Eternal Universe

There are three neutrino species in the standard model, hereafter refereed to as 1, 2, and 3, that we know have mass from atmospheric and solar neutrino oscillation experiments. Furthermore, data from these experiments put constraints on the mass-splittings between these three neutrinos.  From atmospheric experiments we know the mass differences between 2 and 3 is |M223| ~ 1.4x10-3 eV2 and from

... Read more »

Jimenez, R., Kitching, T., Peña-Garay, C., & Verde, L. (2010) Can we measure the neutrino mass hierarchy in the sky?. Journal of Cosmology and Astroparticle Physics, 2010(05), 35-35. DOI: 10.1088/1475-7516/2010/05/035  

  • September 15, 2010
  • 12:16 PM

"Rational" Referees May Hurt The Peer Review Process.

by Joseph Smidt in The Eternal Universe

Those of us who work in academic fields hope that the peer review process in some sense works.  Thurner and Hanel recently studied the effects of one particular entity that may hurt the process: the rational referee.  Here, rational referes to someone who largely accepts or rejects a paper by factoring in how the acceptance or rejection of the paper will impact himself/herself. (To me this is not

... Read more »

  • September 10, 2010
  • 11:58 AM

Did Life Develop Shortly After Big Bang and Get Spread Throughout The Universe?

by Joseph Smidt in The Eternal Universe

I woke up to a very interesting paper by Gibson, Wickramasinghe, and Schild that appeared on the ArXiv last night and suggests that life most likely developed shortly after the big bang and was then spread throughout the universe.   They call this the biological big bang. (And at this point I should say that universe here means the "local universe" that was in casual contact between 2-8 million

... Read more »

Carl H. Gibson, N. Chandra Wickramasinghe, & Rudolph E. Schild. (2010) First life in primordial-planet oceans: the biological big bang. Submitted to International Journal of Astrobiology. arXiv: 1009.1760v1

  • September 7, 2010
  • 01:13 PM

Standard Cosmology Theory Is Confirmed By ACT For Smallest Scales In The Universe.

by Joseph Smidt in The Eternal Universe

It never ceases to amaze me how well standard cosmology theory fits the ever increasing amount of data with precision. The results just released from the Atacama Cosmology Telescope (ACT) confirm that, even on the smallest scales, the predictions of the Lambda CDM universe preceded by an epic of inflation are correct.  This study extracts data for L modes of The CMB between 500 and 10,000.  (For

... Read more »

  • August 18, 2010
  • 11:23 AM

Could The Planck Satellite Discover A New Species Of Neutrino?

by Joseph Smidt in The Eternal Universe

It has been known for some time that the WMAP data is more consistant with the existence of four neutrino species than three. Nevertheless, most cosmologists shrug this off as three is by no means ruled out. However, Hamann et al. 2010 demonstrate that such a dismissal may be a mistake.

It turns out, when WMAP 7 year data is combined with Sloan data, the three neutrino species model is ruled

... Read more »

Jan Hamann, Steen Hannestad, Georg G. Raffelt, Irene Tamborra, & Yvonne Y. Y. Wong. (2010) Cosmology seeking friendship with sterile neutrinos. Eprint. arXiv: 1006.5276v1

  • July 7, 2010
  • 12:46 PM

Massive Early Stars And Molecular Hydrogen Cooling

by Joseph Smidt in The Eternal Universe

The big bang produced only Hydrogen and Helium with trace amounts of Lithium. (For the most part.)  This is a problem for star formation because stars need to be "cool" to form and typically you need heavier elements to help the star cool off.  This is why:

Gravity pulls mass together.  However, as matter gets pulled together it heats up and this heat causes the matter to want to expend again.  

... Read more »

  • June 25, 2010
  • 01:05 PM

A Great History Of The Evidence For Dark Matter.

by Joseph Smidt in The Eternal Universe

In the paper Dark Matter: A Primer Garrett and Dudagives give a nice historical background to the accumulating evidence for dark matter.  Lets go through the history they lay out.

1.  J. H. Oort:  Astronomers have come to tust what is known as the mass to light ratio, M/L, that does a good job telling you what the mass of luminous matter should be based off of the luminosity of that matter.  

... Read more »

Katherine Garrett, & Gintaras Duda. (2010) Dark Matter: A Primer. Eprint. arXiv: 1006.2483v1

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