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.