Astronomy Without A Telescope – Backgrounds


You’ve probably heard of the cosmic microwave background, but it doesn’t stop there. The as-yet-undetectable cosmic neutrino background is out there waiting to give us a view into the first seconds after the Big Bang. Then, looking further forward, there are other backgrounds across the electromagnetic spectrum – all of which contribute to what’s called the extragalactic background light, or EBL.

The EBL is the integrated whole of all light that has ever been radiated by all galaxies across all of time. At least, all of time since stars and galaxies first came into being – which was after the dark ages that followed the release of the cosmic microwave background.

The cosmic microwave background was released around 380,000 years after the Big Bang. The dark ages may have then persisted for another 750 million years, until the first stars and the first galaxies formed.

In the current era, the cosmic microwave background is estimated to make up about sixty percent of the photon density of all background radiation in the visible universe – the remaining forty per cent representing the EBL, that is the radiation contributed by all the stars and galaxies that have appeared since.

This gives some indication of the enormous burst of light that the cosmic microwave background represented, although it has since been red-shifted into almost invisibility over the subsequent 13.7 billion years. The EBL is dominated by optical and infrared backgrounds, the former being starlight and the latter being dust heated by that starlight which emits infrared radiation.

Just like the cosmic microwave background can tell us something about the evolution of the earlier universe, the cosmic infrared background can tell us something about the subsequent evolution of the universe – particularly about the formation of the first galaxies.

The power density of the universe's background radiation plotted over wavelength. The cosmic microwave background, though substantially red-shifted due to its age, still dominates. The remainder, extragalactic background light, is dominated by optical and infrared radiation, which have power densities several orders of magnitude higher than the remaining radiation wavelengths.

The Photodetector Array Camera and Spectrometer (PACS) Evolutionary Probe is a ‘guaranteed time’ project for the Herschel Space Observatory. Guaranteed means there always a certain amount of telescope time dedicated to this project regardless of other priorities. The PACS Evolutionary Probe project, or just PEP, aims to survey the cosmic infrared background in the relatively dust free regions of the sky that include: the Lockman Hole; the Great Observatories Origins Deep Survey (GOODS) fields; and the Cosmic Evolution Survey (COSMOS) field.

The Herschel PEP project is collecting data to enable determination of rest frame radiation of galaxies out to a redshift of about z =3, where you are observing galaxies when the universe was about 3 billion years old. Rest frame radiation means making an estimation of the nature of the radiation emitted by those early galaxies before their radiation was red-shifted by the intervening expansion of the universe.

The data indicate that infrared contributes around half of the total extragalactic background light. But if you just look at the current era of the local universe, infrared only contributes one third. This suggests that more infrared radiation was produced in the distant past, than in the present era.

This may be because earlier galaxies had more dust – while modern galaxies have less. For example, elliptical galaxies have almost no dust and radiate almost no infrared. However, luminous infrared galaxies (LIRGs) radiate strongly in infrared and less so in optical, presumably because they have a high dust content.

Modern era LIRGs may result from galactic mergers which provide a new supply of unbound dust to a galaxy, stimulating new star formation. Nonetheless, these may be roughly analogous to what galaxies in the early universe looked like.

Dustless, elliptical galaxies are probably the evolutionary end-point of an galactic merger, but in the absence of any new material to feed off these galaxies just contain aging stars.

So it seems that having a growing number of elliptical galaxies in your backyard is a sign that you live in a universe that is losing its fresh, infrared flush of youth.

Further reading: Berta et al Building the cosmic infrared background brick by brick with Herschel/PEP

11 Replies to “Astronomy Without A Telescope – Backgrounds”

  1. The neutrino background has its origin from the first 3 seconds of the universe. If we could observe that we would be getting signal from much earlier moments of universe. The neutrinos would be from the decoupling of the GUT gauge-fermion system, which was likely an SO(10) theory, or as I think it is from an E_6 gauge field. I think these neutrinos would likely be sterile with respect to oscillations. The oscillations would largely have stopped, and so with neutrino detection, say the upgrade from Sudbury and Super-K, we might be able to detect this neutrino background. If there is anisotropic structure to it then with several detectors it should be possible to map that out.

    Even earlier would be the detection of gravitons. These have their origins in the first 10^{-20} seconds of the universe. The gravitons will however be very redshifted, and in fact red shifted to the point they have wavelengths comparable to the horizon length of the observable universe. However, these likely formed gravity waves which resulted in anisoptropy of the CMB. In effect non-Gaussian signatures in the CMB serve as our graviton (gravity wave) detector.


    1. Thanks for that. Are gravitons just a wave-particle duality characteristic? That is – are they interchangeable with gravity waves depending on how you measure such things?

      Or are we talking about completely different phenomena?

    2. A graviton is similar to a photon. In the weak field case of gravity waves the nonlinear terms can be dropped. The equation for weak gravity waves is a form similar to the wave equation for photons. The metric g_{ab} = ?_{ab} + h_{ab} is a flat spacetime metric ?_{ab} plus a small perturbation h_{ab} If you put this through the Einstein field equation for a vacuum spacetime it is not too hard to show that this leads to a differential equation for a wave

      (?_z^2 – ?_t^2)h_{ab} = 0

      where for technical reasons the h_{ab} here is a traceless form of the tensor h_{ab}. The difference is that the wave is a tensor field and not a vector field. As a result there are two directions of polarization.

      This equation can be quantized fairly easily. The tensor field is similar to a di-photon in quantum optics, or two entangled photons. This is a weak graviton, not at all a full theory of quantum gravity but “a start” at such. It has all of those strange quantum complementarity principles, such as wave-particle duality,

      Gravitons were produced in the decoupling of gravitation from the grand quantum field at the onset of inflation. During inflation the wavelengths of these gravitons were stretched out enormously. So a coherent stream of gravitons were reduced to a classical gravity wave with a wavelength far larger than the deBroglie wavelength of quantization. These then generated pockets of accumulated material and inhomegeneous or anisotropic distributions. The imprint of these should exist on the CMB as certain nonGaussian modes which are signatures of the two polarization states of the graviton.


      1. Thanks, interesting. I have never heard anyone draw a link between gravitons and gravity waves before (though that may just be me not moving in the right circles).

        It seems likely that we will eventually find evidence of gravity waves – at which time I guess we can say we’ve found evidence of gravitons too.

      2. Gravitons are similar to photons or other gauge particles. In the case of electromagnetism photons mediate the force. Suppose we have two charged particles with momentum p and p’. The Heisenberg uncertainty principle indicates that photons with some momentum uncertainty ?p may pop in and out of the vacuum within some uncertainty in position ?x according to the Heisenberg uncertainty principle ?p?x ~ ?. Then with two charged particles this virtual photon can rob momentum from the particle with momentum p — > p – ?p and transfer this to the particles with momentum p’ — > p’ + ?p. In this way a force is mediated by virtual photons in the vacuum, and so two charged particles exert a force between them. Something similar occurs with gravitons, which is why there is a gravity “force.”

        However, gravity is not really a force as such. Further, this becomes strange when you try to fully quantize spacetime. In effect this involves the propagation of a field with spacetime data on spacetime or itself. This makes things difficult. This weak semi-classical description has some problems with regards to quantizing the entire universe, such as how the universe quantum mechanically tunneled out of the vacuum.


      3. I don’t know much about the math, but in theory it seems like the graviton and Higgs are possibly one in the same. A particle that carries the force that bind matter together seems logical that it in itself could be the matter.

      4. Is the Higgs field the same as a graviton? The idea has been kicked around some, though nothing has come of it in particular. The Higgs field obeys a wave equation, here written with one spatial direction x,

        ?^2?/?x^2 – ?^2?/?t^2 – F(?) = 0

        where F(?) is s force which is of the form F(?) = -?? + ?|?|^2?. This is a cubic force term which has some interesting properties. In one dimension it means there are two points where the force is minimum, and this is a degeneracy of states. A little work with this at F(?) = 0 and the quadratic equation that results should illustrate this. In two space dimensions the potential term has a wine bottom potential appearance. The low energy Higgs field is then an ensemble of waves on this degenerate vacuum in a condensate form. The Higgs field with two components (?_1,?_2) couples to a gauge field one of these components is annihilated (Goldstone absorption) and the remaining part becomes a mass-like term, which I will call h. So for a gauge field A ends up with a term A^†hA ~ m^2A^†A, and fermion particle such as quarks q have a similar term ~ q†hq that are a “mass.”

        As one goes up in energy the Higgs field is restored more as a field uncoupled from other fields. The mass terms attenuate and fields are increasingly massless. At this stage there is a new domain of physics. This really has to in some generic setting hold. Standard QFT does not work beyond about 1TeV in energy. The Higgs mechanism is the obvious fix to this problem. However, the energy scale from transverse momentum scales >~ 1TeV may involve physics that is quite different from what we currently know.

        One of these differences is the idea behind extra large dimensions, along with the prospect for black hole or AdS signatures. The compactification of a particle on a length scale R leads to a mass or energy that scales as ~ 1/R. If R is on the order of the string length this is a huge mass term. However, maybe this compactification scale has a renormalization group flow with the transverse energy of interaction. Then at low energy, here low being around the LHC energy scale, the compactification length might be on the order of {100GeV}^{-1} and which may becomes smaller at high energy interactions or transverse momentum. If there is a renormalization group flow of this sort it could then potentially be the case that at TeV energy scales there are amplitudes or channel processes corresponding to black holes or AdS-gluon chain physics.

        So once the Higgs field is “free” at high enough energy the underlying conformal structure of reality should start to unfold. So the completion of the standard model should also be the start of a foundational physics where extra large dimensions emerge. These emerge at a cut off scale that is at energy scales somewhat higher than the probe energy, where as one scales to high energy approaching the string scale, sometimes called the Hagedorn temperature, the gap between the cut-off energy and the probe energy converges to zero. This has a renormalization group flow, which in its structure is a form of the Navier-Stokes equation for the flow of a fluid. However, this flow is not in space, but in momentum space. Steinhardt and other have demonstrated that some of the solution types of the Einstein field equation are equivalent to solutions of the Navier-Stokes equations. This then leads to a hypothesis that the renormalization group flow for conformal field theory above the Higgs condensate energy is a dual form of the Einstein field equations. The Higgs field is then the switch which turns this on, or is the regulator of that energy gap.


    3. There’s also a preposition ‘duality’ in the first sentence of the fourth paragraph: “… is estimated to make up about of sixty…”

  2. Does anyone see the “face” in that image? Yes, I realize that we are hardwired to recognize faces but I still find it interesting how one just jumped out at me out of the noise.

    1. Don’t know about jumped out. If I search, I can make out a couple of vague circles that might do for eyes and a mouth lower down. Grey alien maybe – or Bigfoot.

    2. Pretty much looks like a Schnauzer to me! But then, I see animals on my stucco ceiling too…


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