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Contents:
  1. chapter and author info
  2. The Particle That Broke a Cosmic Speed Limit
  3. Featured resources
  4. Cosmic Speed Measurement Suggests Dark Energy Mystery - Scientific American

Various other measurements seemed to support their idea, too, leading to their being awarded the Nobel Prize in Physics in Today, most cosmologists believe about two-thirds of the universe is made of dark energy.

chapter and author info

In fact, their absolute brightness varies by about a factor of ten. We found that the evidence for cosmic acceleration had a statistical significance of only three standard deviations, or three sigma, away from zero.

You should not get too excited until you have at least a five-sigma result — certainly when it concerns fundamental physics. In our latest work , published in August this year, we went one step further by accounting for the inhomogeneity of the universe — that is, the uneven distribution of matter. Because of this, expansion is not quite the same in all directions.

In fact, the Milky Way is being pulled in a particular direction at over kilometres per second. It turns out that most of the supernovae studied so far are within this unusual region — the net result being that the evidence for dark energy, previously three sigma, drops even further to two sigma. Even an astronomer would baulk at saying two sigma is significant.

We are moving relative to other objects in the universe, and that biases our measurements. Your view that evidence for dark energy is on shaky ground is very much a minority one within cosmology. Why have you persuaded so few of your colleagues? In other words, they may be suffering from confirmation bias.

For example, several of the key people behind the Large Synoptic Survey Telescope under construction in Chile are particle physicists. But we need to be open to different possibilities. If you liked this interview, please consider sharing it on social media. It may be that life is lurking out there on other planets. Calculations suggest the protons should max out around 0. Heavier nuclei from supernova shock waves are thought to be capable of reaching 0. No one is sure what to make of the discrepancy.

Past the ankle, at around 60 EeV, the line dips toward zero, forming a sort of toe. This is probably the GZK cutoff , the point beyond which cosmic rays can only tarry for so long before losing energy to ambient cosmic microwaves generated by a phase transition in the early universe. From there, the energy spectrum reduces to a trickle of trans-GZK cosmic rays, finally ending, at EeV, with a single data point: the Oh-My-God particle.

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The Particle That Broke a Cosmic Speed Limit

The presence of the GZK cutoff means that the laws of physics are operating as expected. Rather than disproving those laws, trans-GZK cosmic rays probably do originate nearby reaching Earth before ambient microwaves sap their energy. But where, and how? For a maddening 20 years, the particles appeared to come from everywhere and nowhere in particular. But finally a hotspot has developed in the Northern Hemisphere. Could this be the invisible gorilla hurtling bowling balls toward Earth?

Cosmology, Critical Density, Big Crunch, Oscillating Universe, Accelerating Expansion, Dark Energy

The hotspot of trans-GZK cosmic rays, which centers on the constellation Ursa Major, was initially too weak to be taken seriously. Thomson and his team must reach five-sigma certainty to definitively claim a discovery. Already, theorists are treating the hotspot as an anchor for their ideas. The invisible gorilla would materialize.

Meanwhile, some of those other particles are slowly piling up in the sensors of the IceCube detector, a cable-infused, cubic-kilometer block of ice buried beneath the South Pole. For the past four years, IceCube has monitored the rare ice tracks of neutrinos, lightweight elementary particles that usually flit right through matter and thus require immense efforts to detect, but which are produced in abundance from physical processes throughout the universe.

Every so often, cosmic neutrinos interact with atoms and produce radiation as they pass through IceCube; their directions of travel trace a new map of the cosmos that can be compared to the maps of ultrahigh-energy cosmic rays and those of light. In , IceCube scientists reported the observation of the first-ever very-high-energy neutrinos — a pair of 0. Neutrinos have a big advantage over cosmic rays as messengers from the most powerful objects in the universe: Because they are electrically neutral, they move in straight lines.

Of the 54 high-energy neutrinos that IceCube has detected as of its latest analysis , reported in early May , four originate from the vicinity of the cosmic-ray hotspot. Neutrinos can enter the detector after traveling through Earth from the northern sky. Short-lived source candidates such as gamma-ray bursts would be ruled out in favor of stable objects — perhaps a star-forming galaxy with a supermassive black hole at its center.

For now, though, the correlation is very weak. The search will organize around the hotspot.

Featured resources

And all we need to do is collect more data. Kampert, of the Pierre Auger Observatory, is approaching the mystery of ultrahigh-energy cosmic rays from a different direction, by asking: What are they? Another way to determine the cosmological constant is to deduce the expansion of the universe from the redshift of the light from distant sources. This is the way the Nobel-Prize winners made their original discoveries in the late s, and the precision of this method has since been improved.

Cosmic Speed Measurement Suggests Dark Energy Mystery - Scientific American

In addition, there are now multiple ways to make this measurement, where the results are all in general agreement with one another. But these two ways to determine the cosmological constant give results that differ with a statistical significance of 3. There are reasons to be skeptical, because the tension goes away when the finer structures the large multipole moments of the data are omitted. In addition, incorrect foreground subtractions may be continuing to skew the data, as they did in the infamous BICEP2 announcement.

One way of measuring the Universe's expansion history involves going all the way back to the first The other ways don't go backwards nearly as far, but also have a lesser potential to be contaminated by systematic errors. In this case, several modifications of the standard cosmological model have been put forward. They range from additional neutrinos to massive gravitons to actual, bona fide changes in the cosmological constant.

The idea hat the cosmological constant changes from one place to the next is not an appealing option because this tends to screw up the CMB spectrum too much.

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The different ways dark energy could evolve into the future. It's assumed it will remain constant A group of researchers from Spain, for example, claims that they have a stunning 4.


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This claim seems to have been widely ignored, and indeed one should be cautious. Moreover, they fit their model not only to the two above mentioned datasets, but to a whole bunch of others at the same time.


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A couple of cosmologists who I asked about this remarkable result and why it has been ignored complained that the Spanish group's method of data analysis is non-transparent. Any configuration of background points of light -- stars, galaxies or clusters -- will be distorted