The Planck Mission

Yesterday, the Planck Mission released a new map of the cosmic microwave background radiation:

380,000 years after the Big Bang, the Universe cooled down enough for protons and electrons to settle down and combine into hydrogen atoms. Protons and electrons are charged, so back when they were freely zipping around, no light could go very far without getting absorbed and then re-radiated. When they combined into neutral hydrogen atoms, the Universe soon switched to being almost transparent… as it is today. So the light emitted from that time is still visible now!

And it would look like this picture here… if you could see microwaves.

When this light was first emitted, it would have looked white to our eyes, since the temperature of the Universe was about 4000 kelvin. That’s the temperature when half the hydrogen atoms split apart into electrons and protons. 4200 kelvin looks like a fluorescent light; 2800 kelvin like an incandescent bulb, rather yellow.

But as the Universe expanded, this light got stretched out to orange, red, infrared… and finally a dim microwave glow, invisible to human eyes. The average temperature of this glow is very close to absolute zero, but it’s been measured very precisely: 2.725 kelvin.

But the temperature of the glow is not the same in every direction! There are tiny fluctuations! You can see them in this picture. The colors here span a range of ± .0002 kelvin.

These fluctuations are very important, because they were later amplified by gravity, with denser patches of gas collapsing under their own gravitational attraction (thanks in part to dark matter), and becoming even denser… eventually leading to galaxies, stars and planets, you and me.

But where did these fluctuations come from? I suspect they started life as quantum fluctuations in an originally completely homogeneous Universe. Quantum mechanics takes quite a while to explain – but in this theory a situation can be completely symmetrical, yet when you measure it, you get an asymmetrical result. The universe is then a ‘sum’ of worlds where these different results are seen. The overall universe is still symmetrical, but each observer sees just a part: an asymmetrical part.

If you take this seriously, there are other worlds where fluctuations of the cosmic microwave background radiation take all possible patterns… and form galaxies in all possible patterns. So while the universe as we see it is asymmetrical, with galaxies and stars and planets and you and me arranged in a complicated and seemingly arbitrary way, the overall universe is still symmetrical – perfectly homogeneous!

That seems very nice to me. But the great thing is, we can learn more about this, not just by chatting, but by testing theories against ever more precise measurements. The Planck Mission is a great improvement over the Wilkinson Microwave Anisotropy Probe (WMAP), which in turn was a huge improvement over the Cosmic Background Explorer (COBE):

Here is some of what they’ve learned:

• It now seems the Universe is 13.82 ± 0.05 billion years old. This is a bit higher than the previous estimate of 13.77 ± 0.06 billion years, due to the Wilkinson Microwave Anisotropy Probe.

• It now seems the rate at which the universe is expanding, known as Hubble’s constant, is 67.15 ± 1.2 kilometers per second per megaparsec. A megaparsec is roughly 3 million light-years. This is less than earlier estimates using space telescopes, such as NASA’s Spitzer and Hubble.

• It now seems the fraction of mass-energy in the Universe in the form of dark matter is 26.8%, up from 24%. Dark energy is now estimated at 68.3%, down from 71.4%. And normal matter is now estimated at 4.9%, up from 4.6%.

These cosmological parameters, and a bunch more, are estimated here:

Planck 2013 results. XVI. Cosmological parameters.

It’s amazing how we’re getting more and more accurate numbers for these basic facts about our world! But the real surprises lie elsewhere…

A lopsided universe, with a cold spot?

 

The Planck Mission found two big surprises in the cosmic microwave background:

• This radiation is slightly different on opposite sides of the sky! This is not due to the fact that the Earth is moving relative to the average position of galaxies. That fact does make the radiation look hotter in the direction we’re moving. But that produces a simple pattern called a ‘dipole moment’ in the temperature map. If we subtract that out, it seems there are real differences between two sides of the Universe… and they are complex, interesting, and not explained by the usual theories!

• There is a cold spot that seems too big to be caused by chance. If this is for real, it’s the largest thing in the Universe.

Could these anomalies be due to experimental errors, or errors in data analysis? I don’t know! They were already seen by the Wilkinson Microwave Anisotropy Probe; for example, here is WMAP’s picture of the cold spot:

The Planck Mission seems to be seeing them more clearly with its better measurements. Paolo Natoli, from the University of Ferrara writes:

The Planck data call our attention to these anomalies, which are now more important than ever: with data of such quality, we can no longer neglect them as mere artefacts and we must search for an explanation. The anomalies indicate that something might be missing from our current understanding of the Universe. We need to find a model where these peculiar traits are no longer anomalies but features predicted by the model itself.

For a lot more detail, see this paper:

Planck 2013 results. XXIII. Isotropy and statistics of the CMB.

(I apologize for not listing the authors on these papers, but there are hundreds!) Let me paraphrase the abstract for people who want just a little more detail:

Many of these anomalies were previously observed in the Wilkinson Microwave Anisotropy Probe data, and are now confirmed at similar levels of significance (around 3 standard deviations). However, we find little evidence for non-Gaussianity with the exception of a few statistical signatures that seem to be associated with specific anomalies. In particular, we find that the quadrupole-octopole alignment is also connected to a low observed variance of the cosmic microwave background signal. The dipolar power asymmetry is now found to persist to much smaller angular scales, and can be described in the low-frequency regime by a phenomenological dipole modulation model. Finally, it is plausible that some of these features may be reflected in the angular power spectrum of the data which shows a deficit of power on the same scales. Indeed, when the power spectra of two hemispheres defined by a preferred direction are considered separately, one shows evidence for a deficit in power, whilst its opposite contains oscillations between odd and even modes that may be related to the parity violation and phase correlations also detected in the data. Whilst these analyses represent a step forward in building an understanding of the anomalies, a satisfactory explanation based on physically motivated models is still lacking.

If you’re a scientist, your mouth should be watering now… your tongue should be hanging out! If this stuff holds up, it’s amazing, because it would call for real new physics.

I’ve heard that the difference between hemispheres might fit the simplest homogeneous but not isotropic solutions of general relativity, the Bianchi models. However, this is something one should carefully test using statistics… and I’m sure people will start doing this now.

As for the cold spot, the best explanation I can imagine is some sort of mechanism for producing fluctuations very early on… so that these fluctuations would get blown up to enormous size during the inflationary epoch, roughly between 10-36 and 10-32 seconds after the Big Bang. I don’t know what this mechanism would be!

There are also ways of trying to ‘explain away’ the cold spot, but even these seem jaw-droppingly dramatic. For example, an almost empty region 150 megaparsecs (500 million light-years) across would tend to cool down cosmic microwave background radiation coming through it. But it would still be the largest thing in the Universe! And such an unusual void would seem to beg for an explanation of its own.

Particle physics

The Planck Mission also shed a lot of light on particle physics, and especially on inflation. But, it mainly seems to have confirmed what particle physicists already suspected! This makes them rather grumpy, because these days they’re always hoping for something new, and they’re not getting it.

We can see this at Jester’s blog Résonaances, which also gives a very nice, though technical, summary of what the Planck Mission did for particle physics:

From a particle physicist’s point of view the single most interesting observable from Planck is the notorious N_{\mathrm{eff}}. This observable measures the effective number of degrees of freedom with sub-eV mass that coexisted with the photons in the plasma at the time when the CMB was formed (see e.g. my older post for more explanations). The standard model predicts N_{\mathrm{eff}} \approx 3, corresponding to the 3 active neutrinos. Some models beyond the standard model featuring sterile neutrinos, dark photons, or axions could lead to N_{\mathrm{eff}} > 3, not necessarily an integer. For a long time various experimental groups have claimed N_{\mathrm{eff}} much larger than 3, but with an error too large to blow the trumpets. Planck was supposed to sweep the floor and it did. They find

N_{\mathrm{eff}} = 3 \pm 0.5,

that is, no hint of anything interesting going on. The gurgling sound you hear behind the wall is probably your colleague working on sterile neutrinos committing a ritual suicide.

Another number of interest for particle theorists is the sum of neutrino masses. Recall that oscillation experiments tell us only about the mass differences, whereas the absolute neutrino mass scale is still unknown. Neutrino masses larger than 0.1 eV would produce an observable imprint into the CMB. [....] Planck sees no hint of neutrino masses and puts the 95% CL limit at 0.23 eV.

Literally, the most valuable Planck result is the measurement of the spectral index n_s, as it may tip the scale for the Nobel committee to finally hand out the prize for inflation. Simplest models of inflation (e.g., a scalar field φ with a φn potential slowly changing its vacuum expectation value) predicts the spectrum of primordial density fluctuations that is adiabatic (the same in all components) and Gaussian (full information is contained in the 2-point correlation function). Much as previous CMB experiments, Planck does not see any departures from that hypothesis. A more quantitative prediction of simple inflationary models is that the primordial spectrum of fluctuations is almost but not exactly scale-invariant. More precisely, the spectrum is of the form

\displaystyle{ P \sim (k/k_0)^{n_s-1} }

with n_s close to but typically slightly smaller than 1, the size of n_s being dependent on how quickly (i.e. how slowly) the inflaton field rolls down its potential. The previous result from WMAP-9,

n_s=0.972 \pm 0.013

(n_s =0.9608 \pm 0.0080 after combining with other cosmological observables) was already a strong hint of a red-tilted spectrum. The Planck result

n_s = 0.9603 \pm 0.0073

(n_s =0.9608 \pm 0.0054 after combination) pushes the departure of n_s - 1 from zero past the magic 5 sigma significance. This number can of course also be fitted in more complicated models or in alternatives to inflation, but it is nevertheless a strong support for the most trivial version of inflation.

[....]

In summary, the cosmological results from Planck are really impressive. We’re looking into a pretty wide range of complex physical phenomena occurring billions of years ago. And, at the end of the day, we’re getting a perfect description with a fairly simple model. If this is not a moment to cry out “science works bitches”, nothing is. Particle physicists, however, can find little inspiration in the Planck results. For us, what Planck has observed is by no means an almost perfect universe… it’s rather the most boring universe.

I find it hilarious to hear someone complain that the universe is “boring” on a day when astrophysicists say they’ve discovered the universe is lopsided and has a huge cold region, the largest thing ever seen by humans!

However, particle physicists seem so far rather skeptical of these exciting developments. Is this sour grapes, or are they being wisely cautious?

Time, as usual, will tell.

27 Responses to The Planck Mission

  1. Derek Wise says:

    This is exciting stuff — and an excellent description of it, John. I hate voicing an opinion on whether some observation will turn out to be correct or just an error. But, I hope this one is true. It would be nice to have some new input for physics. (Something less disastrous than superluminal neutrinos, anyway; thank goodness that one was false.)

  2. Arrow says:

    Great experiment, but personally I find the “explanations” of the fluctuations unconvincing and unnecessary. We can simply ascribe them to the initial state which is and will remain incomprehensible to us anyway. There is simply no way to scientifically approach the question of how an Universe (=everything that exists) can come into being out of nothing (which cannot even be properly defined in this context). In particular the notion that one initial state is more “natural” than another is completely absurd.

    • John Baez says:

      A fine philosophy. It won’t be you who discovers something interesting by studying these fluctuations, then.

      • Arrow says:

        I’m not against studying those fluctuations, I am against preferring some far-fetched speculative explanation invoking quantum fluctuations and inflation to simply ascribing them to an initial state.

        It’s a matter of taste I guess. However if someone come up with an explanation with a better “explained to postulated” ratio I’d certainly be interested.

        • John Baez says:

          If by “far-fetched speculations” you’re talking about the explanations proposed on the Wikipedia page about the cold spot, here’s what I have to say.

          1) The possibility that it’s a due to a supervoid (an unusually large reason with few galaxies). This doesn’t seem particularly far-fetched, since it sounds like the supervoid would only need to be 10 times bigger in diameter than existing voids… and I’d to know a lot more about cosmology than I do to guess which is more unlikely: a void this big arising due purely to coincidence, or a fluctuation in the microwave background that’s even bigger, again arising due purely to coincidence.

          Either way, I’d want to inquire into possible mechanisms that could generate such big fluctuations noncoincidentally. And that leads us to the next option:

          2) The possibility that such a big fluctuation is due to a ‘cosmic texture’, a remnant of a phase transition in the early Universe. Textures, or more generally topological defects, are things that arise in a lot of quantum field theories (and also condensed matter physics). In my post I already suggested the idea of processes occurring before or during the inflationary epoch, that could create fluctuations which then get ‘blown up’ to enormous sizes. A topological defect is the kind of thing that might do the job. The physics ideas have already been worked out, and they’re waiting to be applied to this kind of problem.

          So, this seems like an line of thought worth exploring. That’s what Neil Turok (director of the Perimeter Institute) and coauthors are trying to in this paper:

          • M. Cruz, N. Turok, P. Vielva, E. Martinez-Gonzalez and M. Hobson, A Cosmic Microwave Background feature consistent with a cosmic texture.

          Abstract. The Cosmic Microwave Background provides our most ancient image of the Universe and our best tool for studying its early evolution. Theories of high energy physics predict the formation of various types of topological defects in the very early universe, including cosmic texture which would generate hot and cold spots in the Cosmic Microwave Background. We show through a Bayesian statistical analysis that the most prominent, 5 degree radius cold spot observed in all-sky images, which is otherwise hard to explain, is compatible with having being caused by a texture. From this model, we constrain the fundamental symmetry breaking energy scale to be φ0 ~ 8.7 × 1015 GeV. If confirmed, this detection of a cosmic defect will probe physics at energies exceeding any conceivable terrestrial experiment.

          3) Laura Mersini-Houghton’s idea that it could be the “imprint of another universe beyond our own, caused by quantum entanglement between universes before they were separated by cosmic inflation.” This seems goofy to me, at least at first glance, for quite a number of reasons. For one thing, I don’t see how quantum entanglement could have this effect.

  3. amarashiki says:

    Fascinating! Anyway, let me points out that the polarization CMB data were not ready and were not released yesterday! Those cold spots seen by WMAP and PLANCK are really disturbing…But we should be aware to search for a new physics explanation BEFORE the final Planck data and analysis be available. Personally, LHC data and Planck data are giving evidences of the following fact:

    1st. The boredom principle: Nature prefers simplicity and “boredom”, i.e., minimalist models. Standard Model rules. Cosmological Standard model LCDM, neglecting those anomalies you mention, confirm the big standards we have being studying all these years. FACT: Standard Model and the LCDM model work yet, even they are slightly more supported with the new data.

    2nd. Fiction-Science ideas like extra dimensions, supersymmetry, and string theory are too naive or completely wrong. No evidence from light KK states, no evidence from SUSY at low energy (everyone in the field is slowly tuning the string scale out of the LHC realm, personally I find that move unelegant), no evidence from SUSY or SUSY partners so far. FACT: no extra dimensions so far, no SUSY so far, no string theory so far. Is the final Feynman’s prophecy (wish) of the “string theory breakdown” coming into scene?

    3rd. Higgs field and inflation. I am aware about the Kaku “affair” in the american TV some days ago. Even if the Higgs field is not the inflaton, it is obvious that if inflation is carried out by an scalar field and there is no additional scalar fields in the EW / TeV scale, we have not many options to find the field triggering the inflationary stage AND the current accelerated expansion (of course we can search for models mimicking a scalar field driven cosmic acceleration but they are not “simple” and Nature seems to be “simple”). Question: how could we determine the number of scalar species in the whole Universe? We have only one: this “SM-Higgs like thing” about 125-127 GeV…Can cosmology determine the number of scalar fields? Of course, Cosmology can determine N_eff of “neutrino like species”, but we need to know if there are more scalar fields and their role…Likely tied to the dark stuff we don’t understand yet…FACT: The simplest inflationary theories seems to be strongly favoured by Planck data but the way in which inflation can arise to the structure and (if confirmed) the observed anomalies is far to be completely understood…When these data be confirmed, we will require some concrete model to reproduce this thing, if possible. I am not sure about the “cold spot” but WMAP and Planck seems to see it…What can it be its meaning?

    4th. Gravitational issues. I am not an expert on gravitational waves/astronomy/astrophysics, but the gravitational lensing data of Planck is at least “striking” to my current knowledge, I am wondering if some hints related to the future detection of gravitational waves could be inferred from some sources.

    BONUS: Globally, LHC+Planck+WMAP9+current null results of DM experiments+Neutrino experiments seems to confirm the SM and LCDM, with only a little window for “surprises”, and the complete mystery of Quantum Gravity yet to come! Simple means boring, but the history of physics show that many boring-like theories (relativity, atomic theory) became interesting with further experimental searches!

    In my opinion, some people should be honest about what is the path to follow now. The hierarchy between the neutrino mass, the electroweak scale (or the proton mass if you want to)/Higgs scale and the Planck scale shows no problem at all and the hierarchy seems to be pointed out with data… In fact, Nature favours this scenario, it seems. 3-energy scales tied to neutrinos/confinement and the Higgs/EW scale and the Planck scale. And it seems that the Higgs field does NOT receive high energy corrections after all. The reason why the Higgs field is light remains yet to be explained (and it is tied to the cosmological constant value), specially now that no SUSY or new physics signal has been revealed. What is wrong with the QFT corrrections to the scalar mass?

  4. Stuart Presnell says:

    Is there an alternative projection of the Planck data that makes it easier to visualise it as an image of the spherical sky around us, like an orthographic projection for example?

    Also, which direction are the axis of the asymmetry, and the cold spot? Are there any directionally-nearby pointers — a constellation or a star — that would help find the right point in the sky? (Not that we could actually *see* that the sky looks .0002 kelvin colder, of course! I’m just curious to know where they are.)

    • John Baez says:

      Is there an alternative projection of the Planck data that makes it easier to visualise it as an image of the spherical sky around us, like an orthographic projection for example?

      The WMAP data looks almost like the Planck data, and it’s been around a lot longer, so you can see it in many versions by doing a Google search for images of the cosmic microwave background. I sort of like this:

      and then there’s this:



      Also, which direction are the axis of the asymmetry, and the cold spot? Are there any directionally-nearby pointers — a constellation or a star — that would help find the right point in the sky?

      Clicking my link about the cold spot reveals that it’s centered at galactic longitude 207.8° and latitude −56.3°. Since most of us don’t have much intuition for galactic coordinates, let’s just say it’s in the constellation Eridanus, in the southern sky.

      The new Planck paper says

      In particular, the power spectrum calculated for a hemisphere centered at (θφ) = (110°, 237°) (in Galactic co-latitude and longitude) was larger than calculated in the opposite hemisphere over the multipole range \ell = 2 to \ell = 40.

      Maybe someone here can translate that into the location of a constellation!

  5. A few comments: firstly, these anomalies are not due to experimental error. The likelihood of both WMAP and Planck making different experimental errors (since they have different methodologies) and still seeing the same artificial artifacts is far smaller than the likelihood that these are just statistical fluctuations within the standard model of the universe.

    Related to that is the second point, which is that none of these anomalies is in itself particularly odd, and therefore interpretation of their significance is quite subjective. WMAP regarded essentially the same data as not significant, Plancks think it is.

    And finally, this is basically the final word on this matter: Planck has essentially seen whatever is possible to see in the CMB (there’s still the polarisation of the CMB, which data Planck sees but has not yet released, but even that is unlikely to tell us anything more about these anomalies).

    • John Baez says:

      Hi! Interesting comments. Since you work on theoretical cosmology I guess you know what you’re talking about. But still, I have some questions.

      The likelihood of both WMAP and Planck making different experimental errors (since they have different methodologies) and still seeing the same artificial artifacts is far smaller than the likelihood that these are just statistical fluctuations within the standard model of the universe.

      I’m mainly worried that they’re not correctly subtracting microwave sources in the Galaxy, or other sources that both WMAP and Planck would see. I don’t have any special expertise in this, but when I see Jester saying this:

      I was a bit surprised by how much emphasis in today’s press conferences was put on the small glitches at low multipoles. It seems that Planck people are also a bit frustrated by the fact that their results are nothing but a triumphant confirmation of old paradigms. Even at the LHC nobody would make a big deal of a 2.5 sigma anomaly, and in the present case we’re in the area of astrophysics where error bars are treated more loosely ;-) Moreover, according to Planck, the quadrupole mode in the fluctuation spectrum is aligned with the ecliptic plane, which suggests some unknown background or pesky systematics at large angular scales.

      … and it makes me worried that some of the subtler effects may be artifacts… even though he too may have no special expertise.

      Related to that is the second point, which is that none of these anomalies is in itself particularly odd…

      Could you explain what that means? Does it mean that their probability is high, given some model or other? If so, where can I find out more about this? Just Planck 2013 results. XXIII. isotropy and statistics of the CMB, or is there something else I should be reading? Maybe some basic stuff on the cold spot and the hemispheric anomaly.

      • Bruce Smith says:

        That Jester quote says “the quadrupole mode in the fluctuation spectrum is aligned with the ecliptic plane”. I read a few press reports about the overall story, and just one of them said the hemispheric anomaly is aligned with the plane of the ecliptic (i.e. of the orbit of the earth around the sun). None of the others addressed its alignment. Can you confirm this more directly? If true, it seems to me very significant and the other press reports ought to point it out more strongly. (Whether it’s an error or new physics, it opens up an additional “more local” set of explanations to look for.)

        Related questions: how well aligned is it? (And how precise an orientation can be defined from this data?) Is there anything else outside our solar system that happens to have the same alignment?

      • Hi John. Subtraction of foregrounds is obviously an issue to be concerned about. However, a lot of careful work has been done on this, so if there are still some foreground effects present, they will have some odd properties. Planck scans the sky in a much wider range of frequencies than WMAP, which makes it better placed to detect foreground sources emitting with a non-blackbody spectrum.

        Regarding Jester’s comment about the alignment with the ecliptic, you might be interested in the opinion of Hans Kristian Eriksen, one of the authors of the Planck papers.

        Finally, when I say the anomalies individually are not particularly odd, I mean that in the standard \LambdaCDM model they are only discrepant at around the 3\sigma level, or odds of 1 in 100. At that level, they’re odd enough to remark on, but unless someone can come up with an alternative model which can explain them, they may ultimately be nothing more than curiosities, simply the effect of finite sampling of a random variable.

        If you want a different take on the anomalies, you could read the WMAP paper “Are there cosmic microwave background anomalies?”, which draws very different conclusions from essentially the same data.

  6. domenico says:

    Thank you for the description of the Plank results: it is the best reasoned synthesis of the research, and problems.

    I have not heard, until now, of perfect (in general realm) symmetric explosions (cold zone can be natural); and we see only half of the Big Bang (the other region can be specular: hot-cold reversal).

    I think that the initial Big Bang matter production can be related to the mass velocity (curvature of the space or rather Dirac sea), so that the density mass can be related to the velocity (cold zone with low density and low velocity).

    Saluti

    DOmenico

    • domenico says:

      I am thinking to an analysis of cold spot description.
      There is a simple discrimination between a curvature bubble near the Big Bang (cosmic ripple or a high density zone), and an asimmetric Big Bang.
      A statistical study of the density of optical galaxies near the cold spot can distinguish the two theories: if there is not density difference in simultaneous galaxies then the Big Bang is uniform (curvature ball), if there is an inhomogeneity in the galaxies distribution then the Big Bang is inhomogeneous.
      The density variation (of the galaxies) in the time can give an information of the curvature ball, or the inhomogeneities in the Big Bang; this reminds me the Liouville’s theorem where we see section of the flow in different times.

      Saluti

      DOmenico

  7. Thank you for this great summary John. I have a question though.

    Protons and electrons are charged, so back when they were freely zipping around, no light could go very far without getting absorbed and then re-radiated.

    But photons are not charged, so what charge has to do with anything ?

    Why it would have been easier for light to pass if protons and electrons were not charged ?

    • John Baez says:

      Light is only emitted and absorbed by electrically charged particles. Photons are not charged, so they don’t absorb or emit light. They are light, but that’s completely different!

      As a result, a gas of freely moving charged particles is a great way to stop light. This is why most metals are good at stopping light: they contain what amounts to a gas of freely moving electrons. A hot ‘plasma’ of electrons and protons is also good at stopping light. When these cool down to the point of forming electrically neutral hydrogen atoms, we get a gas that’s much closer to transparent.

      Of course light can still be absorbed by hydrogen, but mostly this happens when its frequency is right to push an electron in the hydrogen from one energy level to another: light at these particular frequencies can ‘see’ that while the hydrogen atom is electrically neutral overall, it’s made of charged particles.

      By the way, the basic interaction in quantum electrodynamics is this, where the straight line is a charged particle and the wiggly one is a photon:

      All interactions between matter and electromagnetic fields, or light, can ultimately be traced back to this interaction.

      This picture is also a kind of cryptic notation for a bunch of math: the math of quantum electrodynamics.

      • Ok, i meant to say that photons, being uncharged, are not sensitive to charge so can’t be attracted or repelled by charged particles. So with no obvious force exchanged between photons and protons/electrons I couldn’t figure out the exact mechanism involved.

        And I did not know that they can only be emitted or absorbed by charged particles, interesting. So can photons pass through neutrons? or they just bounce off them? In the first case neutron stars, or at least their core, should be transparent, somehow i have hard time to believe that …

        • John Baez says:

          And I did not know that they can only be emitted or absorbed by charged particles, interesting.

          You’ve probably seen Maxwell’s equations, which say at the classical level how charges and currents produce electromagnetic fields… and maybe also the Lorentz force law, which say how electromagnetic fields push on charged particles. When you study this stuff taking quantum mechanics into account you get quantum electrodynamics, which says it’s all about photons interacting with charged particles.

          So can photons pass through neutrons?

          Neutrons are made of quarks, which are charged, so just like neutral hydrogen atoms they will interact with photons if you’re clever enough to probe this internal structure. But they should be vastly closer to ‘invisible’ than protons or electrons.

          or they just bounce off them?

          ‘Bouncing off’ counts as an interaction.

          In the first case neutron stars, or at least their core, should be transparent, somehow i have hard time to believe that …

          Neutron stars are full of stuff besides neutrons, and a lot of this stuff is electrically charged:

          Click for details.

        • You are right, i have studied both Maxwell equations and the Lorenz force, and in fact it, under this light, it makes perfect sense that electromagnetic fields are stopped by (or anyway interact with) charged particles and do not interact with uncharged matter.

          For some reason i was thinking of photons more as little balls than as electromagnetic fields, even though i know they are.

          Ok. I got it. Thanks for the physics refresher :)

  8. Allen Knutson says:

    I’m having philosophical trouble with the phrase “the largest thing ever seen by humans”. Maybe it’s with the word “thing”. (Obviously, the picture as a whole is larger than the spot within it. And just this morning, I opened my eyes and saw the universe.) Can you give me a sense in which this cold spot is, or might be, “one thing”?

    Like, the hole left because a supernova pushed everything out of some neighborhood? Is that a reasonable sort of guess?

    • John Baez says:

      This cold spot is much much much much much much much much much much much much much much much much much much much much much much much much much much much much much much much much much too large to be a hole punched out by a supernova.

      The Local Bubble in which we live was probably punched out by a supernova. It fits in our galaxy and it’s about 300 light years across.

      We don’t know how exactly how big the cold spot is, since we don’t know how far away it is. We know for sure that has an angular size 5° as seen from here. So if we could see it, it would look 6 times the angular size of the Moon!

      One theory for the cold spot is that it’s a region free of galaxies that’s at least 500,000,000 light years across. This (purely hypothetical!) region is called the Eridanus Supervoid or simply the Great Void. There are also other voids known, but the diameter of this one would be about 10 times bigger.

      Voids cause the cosmic microwave background shining through them to be cooled down, thanks to the late-time integrated Sachs–Wolfe effect. Photons leaving denser patches of the universe and climbing against gravity into a void lose energy. When they leave the void and fall back down into denser regions they regain energy, but the Universe has expanded by then, so they don’t regain as much as they lost.

      Another theory is that the cold spot is actually a portion of the universe that was cooler in the first place, i.e. when the microwave background radiation was emitted about 380,000 years after the Big Bang. For this to be true it would need to be even larger, since it would be much farther away.

      I would prefer to postpone the question of whether the cold patch truly deserves to be called a ‘thing’. The fact that we’re talking about it suggests it’s a bit thing-ish. But someday we may know more about whether it’s a ‘coincidental fluctuation’ or an example of an interesting process that creates such cold spots.

      I’d feel happy calling superclusters ‘things’ because while they may have arisen by coincidence, they become gravitationally bound and hang together like galaxies or clusters. We live in a supercluster that’s about 100,000,000 light years across.

      I wasn’t counting the whole universe as a ‘thing’, but if you do, that’s obviously the largest thing… and the fun question becomes what’s the second largest thing.

      • Apparently one explanation for the cold spot is that it could be the sign of a collision with another bubble universe.

        it is not really clear, however, how to reconcile the above with the fact that, as you can read from wikipedia:

        if it is assumed that inflation began about 10^−37 seconds after the Big Bang, then with the plausible assumption that the size of the universe at this time was approximately equal to the speed of light times its age, that would suggest that at present the entire universe’s size is at least 10^23 times larger than the size of the observable universe.

        So our observable universe just happens to be at the border between our bubble and another one ? I don’t know, it seems unlikely, and above all not really clear, (to me at least) …

  9. That is fascinating! Thank you.

  10. Thanks for the overview!

    “It now seems the Universe is 13.82 ± 0.05 billion years old. This is a bit higher than the previous estimate of 13.77 ± 0.06 billion years,”

    Picking nits, it should be the 13.80 +/- 0.04 billion year consolidated (Planck thermal + lensing + WMAP + BAO) age vs WMAP’s consolidated 13.77 figure.

    • Arrow says:

      Very nice, though admittedly the Ghz channels showing our Galaxy look much more interesting to the naked eye than the CMB blobs.

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