Do we live in a special place - Part 2

 

A closely related question to whether we live in a special (high or low density) place in the universe is whether the universe looks the same in all directions.   In a homogeneous universe, everything should look the same in all directions.   This homogeneity has been the default assumption for many years.  For example, IceCube studies of astrophysical neutrinos assume that the flux is the same from all directions.   

 

However, data is beginning to challenge this homogeneity.   Moderate energy (10¹⁵ eV = 1 PeV to 10¹⁷ eV =1 00 PeV) cosmic rays exhibit small density fluctuations at large length scales.   This may be due to the motion of the Earth through the galaxy, so only represent local inhomogeneities.

 

At higher energies, though, the situation is more interesting.  At energies above 10¹⁹ eV, cosmic-ray protons are thought to travel in relatively straight lines, with only limited magnetic deflection in galactic magnetic fields.  So, any observed anisotrophy could be a sign of inhomogeneity at supra-galactic distances.   

 

The two main experiments studying these ultra-high energy cosmic-rays are the Pierre Auger Observatory (PAO) and the Telescope Array (TA), respectively located in the Southern and Northern hemispheres – Malargue, Argentina and Utah, the United States respectively.  These are large arrays – the Pierre Auger Observatory encompasses 3,000 km² (1200 mi²), so can detect very rare, very energetic events.   These two experiments have seen long-standing disagreements in both the spectrum and the composition of the highest energy cosmic rays.   Above energies of about 30 EeV (10¹⁸ eV = 1 EeV) the Auger experiment sees a significant shift toward a heavier cosmic-ray composition (mostly iron,rather than mostly protons) with increasing energy.  The mass is inferred by observing the development of  the cosmic-ray air showers; iron interacts more strongly, so its showers peak higher in the atmosphere (see picture above, from the TA Collaboration).    In contrast, the TA prefers a composition that is mostly lighter elements; the TA also prefers a higher cosmic-ray flux than the Auger Observatory. 

 

The tension between the two experiments has been known for some time, and the two collaborations have tried hard to understand the differences. One intriguing possibility that is gaining some traction is that the difference is because they are observing different portions of the sky, and seeing different contributions from different sources.   This is supported by the emerging evidence for directional anisotropihes in the measurements from the two experiments (see some recent Auger data from a combined anisotrophy study below. In retrospect, this is not too surprising, but it does represent a shift in thinking for many astrophysicists.   

 

 

Do we live in a special place (in the universe)?

Up close, the universe is very inhomogeneous.  There is the odd very-dense star or planet, but most of it is mostly empty space - vacuum.   But, as we zoom out, we expect different large chunks of space to be similar.  These chunks must be very large, because we know that matter clusters in solar systems, galaxies, and even galactic clusters.    

If the universe is indeed homogenous, with matter evenly distributed, then one might think that there is no reason to think our local neighborhood is special.  But, there are a couple of reasons to think that this isn't true.  The first is the anthropic principle.   We live in a place that supports human life - on a planet orbiting a star.  That  puts us in a galaxy which is in a galactic supercluster.   There may be isolated stars not in galaxies (ejected from galaxies, or ?), but they are rare, and the odds favor stars in galaxies.  The fact that we are in a galactic supercluster means we are surrounded by more matter than a random point in the universe, at least unless we average over a very large volume.   

This discussion is important for astrophysicists - especially cosmic-ray enthusiasts because not everything we study comes from far away.  Charged-particle cosmic rays (protons or heavier particles) come mostly from our galaxy, with the most energetic ones likely coming from other nearby galaxies.   Gamma-rays also mostly come from our galaxy.    In contrast, neutrinos can come from far more distant sources.  So, if we want to compare the neutrino rate with the extremely high-energy (i. e. extragalactic) cosmic-ray rate, we have to account for the fact that the cosmic-rays come from a higher-density region of space (i. e. our relatively local neighborhood) than the neutrinos, which come from a much larger volume.   

That said, it is not easy to quantify the increase in density,  since our measurement methods necessarily vary with distance.  A 2009 paper, Andrea Silvestri and Steve Barwick (both at UC Irvine) looked at this effect.  The published version, in Phys. Rev. D is available here, while the freely available arXiv version is available here. Silvestri and Barwick argued that, for rare neutrino sources (if they are rare, then they are well separated, and likely far from us), more than 5 Mpc (megaparsecs) away, there is no anthropic increase in density, but, at smaller distances, the density increase is significant, about 5.3.  Other groups have had different somewhat different estimates;  As our neutrino measurements become more precise, it will be necessary to investigate this bias in more detail.

In my next post, I will discuss a closely related question: does the universe look the same in all directions?

The most energetic neutrino

 

Although IceCube is still the largest neutrino telescope around, it is not the only game in town.   The KM3NeT ARCA telescope is now large enough to study astrophysical neutrinos.  ARCA is located in the Mediterranean, of the coast of Nice, France.  It uses seawater as its optical medium, instead of ice; this has plusses and minuses.  

At the Neutrino 2024 conference in Milan, Joao Coelho (from APC - Paris) presented some initial KM3NeT results.  They have observed an enormously energetic event - far more energetic than any neutrino that IceCube has seen.  The event - seen above -  lit up a fair portion of their detector.  Although the collaboration is still working on a paper on the event - and so has been rather tight-lipped about the details, Joao did talk about some early work estimating its energy.  The Collaboration estimated the energy by comparing the observed event with simulated events of different energies.  

The plot below compares the number of hit photomultiplier tubes (PMTs) for the event with several simulations.  It is important to remember that the scale here is not linear; there are only a certain number of PMTs in the detector.  The deposited energy appears considerably higher 10 PeV - probably 20-40 PeV if one ignores the non-linearity, and higher with it.  This is for the muon energy; the neutrino energy will be higher to much higher, depending on how far away the neutrino interaction is from the active volume and how much energy the departing muon carried off.

The event appears to come from slightly below the horizon.  The collaboration has not discussed possible backgrounds.  The only obvious background in this energy range would be a very energetic muon bundle from a cosmic-ray air shower.   However, this would require a significant misreconstruction - large enough to seem unlikely. If one looks carefully (see the top figure), one can see evidence of stochastic (non-uniform) energy loss, as expected from high-energy muons.  

 Without knowing the energy better, it is hard to tell how compatible this event is with the IceCube limits on extremely-high energy (EHE) neutrinos, but there is at least some statistical tension.    Still, this observation is great news for all high-energy neutrino enthusiasts.

The rock from another world: ʻOumuamua

 

 SETI - the search for extra-terrestrial intelligence and exobiology - the search for extra-terrestrial life are two of the most interesting scientific endeavors of our generation.  The odds of finding extra-terrestrial life may be low, but, increasingly, they seem not that low.   As we learn more about the universe, the likelihood of finding planets with conditions that could harbor life seems higher and higher.  Over the past 20 years, we have learned that planets around other solar systems are common, and that, probably, there are planets in the 'habitable zone,' where liquid water can exist.

One recent event has further stirred interest.  A large rock - named 'Oumuamua,  Hawaiian for scout - flew by the Earth in 2017.  It attracted enormous attention, for several reasons.  First, it was moving very fast - too fast to be in orbit around the Sun.  So, it most likely came from outside the solar system, making it the first interstellar object that we have studied.  Second, it was exhibiting signs of acceleration, although some scientists attributed this to natural outgassing as the object was warmed by the Sun. 

Second, as the picture above (a NASA artists conception, based on optical and radar imaging) it was long and thin, measuring between 300 and 3,000 feet long, with a width and thickness between 115 and 550 feet.  This is not as expected for natural bodies.  The large objects (planets and planetoids) that we can see are fairly round, and, while smaller objects like asteroids and the solid parts of comets are fairly irregular, they are not that irregular.     

 All this led to speculation that 'Oumuamua might be an alien spacecraft visiting our solar system.  There are reasons to wonder - although a gravitational slingshot acceleration in another solar system could possibly explain the acceleration, the probability of an object from another solar system randomly coming so close to Earth (85 times the distance to the Moon) is very very low, unless these objects are very common.  but, it seems hard to explain how they could be naturally accelerated to such high energies, so it seems unlikely that they are common.

These observations of course spurred searches for similar objects.  One interesting search started by looking for similar high-velocity objects that might have been spotted by early-warning radars.  One such trace from 2014 appeared to show a high velocity object, consistent with a diameter around 1.5 m hitting the Earth, near Papua New Guinea.  Harvard astronomer Avi Loeb assembled a team to search for debris from that meteorite.  The team has presented evidence of tiny metallic spherules with 'a chemical composition of unknown origin.'   This has been hotly debated, and other groups have claimed that the spherules are from coal ash.   The debate continues, but, extra-ordinary claims require extra-ordinary proof, so it is, at-best, premature to bet on the extra-terrestrial origin.  

But, the unusual characteristics of 'Oumuamua are well supported by multiple observations, and are well accepted in the scientific community.  Clearly something very interesting passed by, and we as a community should be thinking broadly about follow-up studies.

Citizen Scientists - and neutrinos

Citizen-scientists have a long history; amateur astronomers have made many important discoveries, and, although opinions are mixed  amateur archaeologists have brought many important sites to the attention of professionals.  Now, citizen-science is moving into a new area: neutrino astrophysics.

Although neutrino telescopes are far beyond the reach of amateur scientists, their data is not.  IceCube has enlisted the help of interested amateurs to help with a difficult pattern-recognition problem: classifying the types of neutrino interactions that IceCube sees.  The figure above lists the different types classifications that are currently being considered.   Distinguishing these classes of events is difficult for computer algorithms, but generally easier for people. 

The program, called "Name that Neutrino" is hosted on Zooniverse, a web platform designed for citizen-science applications.  A recent paper, "Citizen Science for IceCube: Name that Neutrino," discusses the program, and reports on the results of classifications by more than 1,800 volunteers.  

All-in-all, a great way to teach science enthusiasts about IceCube and neutrino astronomy.

The mystery of the most energetic cosmic rays deepens

 Recently, the Telescope Array (TA) Collaboration reported on the observation of a cosmic ray with enormous energy: 244 exa-electron volts (EeV, 2.44*10²⁰ eV), or roughly 40 Joules.  This is one of a handful of events seen in this energy range - with an energy roughly comparable to a well hit tennis ball.   Although this is not the most energetic particle ever seen, it is pretty close.  

Fig. 1. A skymap, showing the best-estimate directions for the event, after correcting for expected deflection in galactic and extra-galactic magnetic fields.   The labelled circles are the best estimates for different assumptions about nuclear composition, from protons (p) through iron (Fe) for two different models of the galactic magnetic fields - PT2011 and JF2012 which predicts larger deflections .  The active galaxy PKS1717+177 is a flaring source, but it is 600 Mpc away - probably too far to be the source of this event.  Also shown is a broad hotspot previously seen by TA at lower energies, along  with the galactic plane (the solid line), with the galactic center also indicated.

 

 The intriguing thing about this event is its direction.  The arrival direction was measured to better than 1 degree.  Fig. 1 (above) shows the predicted arrival direction.   At these energies, cosmic rays are not expected to bend very much in galactic and intergalactic magnetic fields.   The TA Collaboration calculated the expected bending for different hypotheses about the nuclear species of the incident particle, from protons to iron, and accounted for that bending.  Then, they looked in that general direction, and found nothing that seemed likely to be able to accelerate particles to those energies. Because 250 EeV particles can interact with the cosmic microwave background radiation and lose energy, their range is limited to 10-30 megaparsecs (Mpc), depending on nuclear species.  The active galaxy PKS1717+177 was considered as a source, but, at 600 Mpc distance, is too far to be a likely source.

This is a finite volume for a possible source, and it was devoid of 'interesting' objects.  'Interesting objects' include active galactic nuclei (galaxies with supermassive black holes at their center, with significant accretion which leads to a relativistic jet) and other sites that may contain the ingredients for a powerful particle accelerator.

There are a couple of possible explanations for the lack of an apparent source - all interesting.  It may be that we need to expand our definition of what is 'interesting' here - the accelerating sites are something that we have not thought of.  Or, maybe, the accelerators do not leave obvious other traces, or are distributed in space.  Or, possibly, the galactic and/or intergalactic magnetic fields are significantly larger than we expect.   

As the name implies, the Telescope Array is an array of surface scintillator detectors (to detect charged particles in cosmic ray air showers) and fluorescence detector telescopes to detect the fluorescence from nitrogen in the air as these charged particles propagate through the atmosphere.

The paper was published in Science, but is also freely available on the arXiv. 

Neutrinos from our galaxy

 IceCube has found a new source of neutrinos - our own Milky Way galaxy, with a significance of 4.5 sigma!   This observation was published in a recent paper in Science; a freely available version is available on the arXiv.  This study is both technically and scientifically very different from the two observations of neutrinos from active galactic nuclei (AGN) that IceCube previously observed.

Earth is within the Milky Way, which is, from our observation point, largely a plane in space so the source surrounds us.  High-energy gamma-rays have been observed coming from the Milky Way  The line has a width of a couple of degrees, depending on how you define the width.  This is very different from other galaxies, which are point sources (or close to that); the different geometry calls for a different analysis technique.  Instead of using muons from muon-neutrinos, this study used `cascades,' which come from electron-neutrinos, and neutral-current interactions of all neutrino flavors.  The advantage of using cascades is that the background of atmospheric neutrinos is much lower, so the signal:noise ratio is higher.  The disadvantage is that the angular resolution isn't nearly as good.  However, IceCube used a machine-learning technique, a convolutional neural network (CNN), to determine cascade directions.   A CNN works in a roughly similar manner to our own brains, with neuron-like processing steps that looked at the light deposition in IceCube's sensors.  This approach gives a resolution that is about two times better than previous cascade directional studies, lessening the difference with nu_mu's.  And, it used many more events.  since the Milky Way is not a point source, the angular resolution is less important.   The figure below shows how the analysis was done:

 

The Milky Way, seen in different ways.  The galactic center is in the middle.  The images extend +/- 15 degrees from the galactic plane., and cover the full 360 degree panorama.   The top panel is a composite optical image of the Milky Way.  The second panel down show the Milky Way as seen by the Fermi satellite Large Area Telescope, using photons with energies above 1 GeV.  The third panel shows a template developed from the Fermi data, assuming that the photons come from π⁰ decays. The fourth panel shows a template for what IceCube should see, after accounting for angular resolution and other detector effects.  Finally, the bottom panel shows the neutrino observations.

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The other interesting thing about the neutrinos from the Milky Way is that we are in the galactic plane.  This is very different from the previous observations, of neutrinos from NGC1068 and TXS0506, both of which are AGNs, with considerable high-energy activity.   We are also observing them from relatively close to their axis of rotation, where high-energy emission is more likely. In contrast, we are in the plane of the Milky Way, and it appears that the neutrinos are coming from many directions.  

IceCube found that the neutrino emission was consistent with the pattern of photon emission, assuming that the photons came from pi^0 and neutrinos come from charged pions.  However, the measured neutrino flux was considerably higher than one would expect based on pi^0 extrapolations.  There could be several reasons for this, including photon absorption en-route to Earth.    The neutrinos and pi^0 might be produced in sources, of when high-energy cosmic-rays interact with atoms or dust while moving around the galaxy.    However, a very recent new search for emission from a number of known sources of energetic (TeV) gamma-rays did not find evidence for an excess of neutrinos. In short, this is an important observation that raises many interesting new questions.