False color image of the Lockman-hole area of the sky at infrared wavelengths as imaged by the Herschel Space Observatory. Credit: ESA/SPIRE Consortium/HerMES Consortium
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When it comes to forming stars, the size of a galaxy does matter, according to research out today in the online version of Nature.
But it doesn’t have to be as massive as we once thought.
Alexandre Amblard, an astrophysicist at the University of California, Irvine, and his colleagues used new data from the Herschel Space Observatory to peer into Lockman Hole area of the sky, where extragalactic light comes from star-forming galaxies out of reach for even the world’s most powerful telescopes.
The Lockman Hole is a patch of the sky, 15 square degrees, lying roughly between the pointer stars of the Big Dipper.
Called submillimetre galaxies, the study subjects emit light at wavelengths between the radio and infrared parts of the spectrum, so studying them requires novel approaches borrowing from both radio and optical astronomy. The galaxies by themselves are too blurry to be resolved with individual far-infrared telescopes – but their average properties can be observed and analyzed, which is exactly what Amblard and his colleagues did.
The authors measured variations in the intensity of extragalactic light at far-infrared wavelengths, and derived statistics for the level of clustering of light halos. They assume that the clustering reflects the underlying distribution of dark matter, and fit the data to a halo model of galaxy formation, which connects the spatial distribution of galaxies in the Universe to that of dark matter.
Distribution of dark matter when the Universe was about 3 billion years old, obtained from a numerical simulation of galaxy formation. The left panel displays the continuous distribution of dark matter particles, showing the typical wispy structure of the cosmic web, with a network of sheets and filaments, while the right panel highlights the dark matter halos representing the most efficient cosmic sites for the formation of star-bursting galaxies with a minimum dark matter halo mass of 300 billion times that of the Sun. Credit: VIRGO Consortium/Alexandre Amblard/ESA
Amblard and his colleagues discovered an enormous fact: the ‘haloes’ of dark matter that surround the Universe’s most active star-forming galaxies are each more massive than about 300 billion solar masses.
What’s even more interesting is that the new threshold for star formation is actually smaller than some previous estimates.
“I think there was one prediction that put the number around 5000 billion times that of the sun, but that was just a prediction from a theory of galaxy formation.“ said Asantha Cooray, also an astrophysicist at UC Irvine and second author on the new paper. “The general consensus was that it may be between 100 to 1000 billion times the sun. We now have a more precise answer from this work.”
Cooray said he’s most excited “that we can look at a detailed image of the sky showing distant, star-forming galaxies and infer not only details about the stars and gas in those galaxies but also about the amount of dark matter needed to form such galaxies. Beyond inferring the presence, we still don’t know exactly what dark matter is.”
The results appear online ahead of print today on Nature’s website.
Artist's conception of the event horizon of a black hole. Credit: Victor de Schwanberg/Science Photo Library
Ever since scientists first discovered the existence of black holes in our universe, we have all wondered: what could possibly exist beyond the veil of that terrible void? In addition, ever since the theory of General Relativity was first proposed, scientists have been forced to wonder, what could have existed before the birth of the Universe – i.e. before the Big Bang?
Interestingly enough, these two questions have come to be resolved (after a fashion) with the theoretical existence of something known as a Gravitational Singularity – a point in space-time where the laws of physics as we know them break down. And while there remain challenges and unresolved issues about this theory, many scientists believe that beneath veil of an event horizon, and at the beginning of the Universe, this was what existed.
Definition:
In scientific terms, a gravitational singularity (or space-time singularity) is a location where the quantities that are used to measure the gravitational field become infinite in a way that does not depend on the coordinate system. In other words, it is a point in which all physical laws are indistinguishable from one another, where space and time are no longer interrelated realities, but merge indistinguishably and cease to have any independent meaning.
This artist’s impression depicts a rapidly spinning supermassive black hole surrounded by an accretion disc. Credit: ESA/Hubble, ESO, M. Kornmesse
Origin of Theory:
Singularities were first predicated as a result of Einstein’s Theory of General Relativity, which resulted in the theoretical existence of black holes. In essence, the theory predicted that any star reaching beyond a certain point in its mass (aka. the Schwarzschild Radius) would exert a gravitational force so intense that it would collapse.
At this point, nothing would be capable of escaping its surface, including light. This is due to the fact the gravitational force would exceed the speed of light in vacuum – 299,792,458 meters per second (1,079,252,848.8 km/h; 670,616,629 mph).
This phenomena is known as the Chandrasekhar Limit, named after the Indian astrophysicist Subrahmanyan Chandrasekhar, who proposed it in 1930. At present, the accepted value of this limit is believed to be 1.39 Solar Masses (i.e. 1.39 times the mass of our Sun), which works out to a whopping 2.765 x 1030 kg (or 2,765 trillion trillion metric tons).
Another aspect of modern General Relativity is that at the time of the Big Bang (i.e. the initial state of the Universe) was a singularity. Roger Penrose and Stephen Hawking both developed theories that attempted to answer how gravitation could produce singularities, which eventually merged together to be known as the Penrose–Hawking Singularity Theorems.
The Big Bang Theory: A history of the Universe starting from a singularity and expanding ever since. Credit: grandunificationtheory.com
According to the Penrose Singularity Theorem, which he proposed in 1965, a time-like singularity will occur within a black hole whenever matter reaches certain energy conditions. At this point, the curvature of space-time within the black hole becomes infinite, thus turning it into a trapped surface where time ceases to function.
The Hawking Singularity Theorem added to this by stating that a space-like singularity can occur when matter is forcibly compressed to a point, causing the rules that govern matter to break down. Hawking traced this back in time to the Big Bang, which he claimed was a point of infinite density. However, Hawking later revised this to claim that general relativity breaks down at times prior to the Big Bang, and hence no singularity could be predicted by it.
Some more recent proposals also suggest that the Universe did not begin as a singularity. These includes theories like Loop Quantum Gravity, which attempts to unify the laws of quantum physics with gravity. This theory states that, due to quantum gravity effects, there is a minimum distance beyond which gravity no longer continues to increase, or that interpenetrating particle waves mask gravitational effects that would be felt at a distance.
Types of Singularities:
The two most important types of space-time singularities are known as Curvature Singularities and Conical Singularities. Singularities can also be divided according to whether they are covered by an event horizon or not. In the case of the former, you have the Curvature and Conical; whereas in the latter, you have what are known as Naked Singularities.
A Curvature Singularity is best exemplified by a black hole. At the center of a black hole, space-time becomes a one-dimensional point which contains a huge mass. As a result, gravity become infinite and space-time curves infinitely, and the laws of physics as we know them cease to function.
Conical singularities occur when there is a point where the limit of every general covariance quantity is finite. In this case, space-time looks like a cone around this point, where the singularity is located at the tip of the cone. An example of such a conical singularity is a cosmic string, a type of hypothetical one-dimensional point that is believed to have formed during the early Universe.
And, as mentioned, there is the Naked Singularity, a type of singularity which is not hidden behind an event horizon. These were first discovered in 1991 by Shapiro and Teukolsky using computer simulations of a rotating plane of dust that indicated that General Relativity might allow for “naked” singularities.
In this case, what actually transpires within a black hole (i.e. its singularity) would be visible. Such a singularity would theoretically be what existed prior to the Big Bang. The key word here is theoretical, as it remains a mystery what these objects would look like.
For the moment, singularities and what actually lies beneath the veil of a black hole remains a mystery. As time goes on, it is hoped that astronomers will be able to study black holes in greater detail. It is also hoped that in the coming decades, scientists will find a way to merge the principles of quantum mechanics with gravity, and that this will shed further light on how this mysterious force operates.
Artist concept of matter swirling around a black hole. (NASA/Dana Berry/SkyWorks Digital)
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Imagine a spinning black hole so colossal and so powerful that it kicks photons, the basic units of light, and sends them careening thousands of light years through space. Some of the photons make it to Earth. Scientists are announcing in the journal NaturePhysics today that those well-traveled photons still carry the signature of that colossal jolt, as a distortion in the way they move. The disruption is like a long-distance missive from the black hole itself, containing information about its size and the speed of its spin.
The researchers say the jostled photons are key to unraveling the theory that predicts black holes in the first place.
“It is rare in general-relativity research that a new phenomenon is discovered that allows us to test the theory further,” says Martin Bojowald, a Penn State physics professor and author of a News & Views article that accompanies the study.
Black holes are so gravitationally powerful that they distort nearby matter and even space and time. Called framedragging, the phenomenon can be detected by sensitive gyroscopes on satellites, Bojowald notes.
Lead study author Fabrizio Tamburini, an astronomer at the University of Padova (Padua) in Italy, and his colleagues have calculated that rotating spacetime can impart to light an intrinsic form of orbital angular momentum distinct from its spin. The authors suggest visualizing this as non-planar wavefronts of this twisted light like a cylindrical spiral staircase, centered around the light beam.
“The intensity pattern of twisted light transverse to the beam shows a dark spot in the middle — where no one would walk on the staircase — surrounded by concentric circles,” they write. “The twisting of a pure [orbital angular momentum] mode can be seen in interference patterns.” They say researchers need between 10,000 and 100,000 photons to piece a black hole’s story together.
And telescopes need some kind of 3D (or holographic) vision in order to see the corkscrews in the light waves they receive, Bojowald said: “If a telescope can zoom in sufficiently closely, one can be sure that all 10,000-100,000 photons come from the accretion disk rather than from other stars farther away. So the magnification of the telescope will be a crucial factor.”
He believes, based on a rough calculation, that “a star like the sun as far away as the center of the Milky Way would have to be observed for less than a year. So it is not going to be a direct image, but one would not have to wait very long.”
Study co-author Bo Thidé, a professor and program director at the Swedish Institute of Space Physics, said a year may be conservative, even in the case of a small rotation and a need for up to 100,000 photons.
“But who knows,” he said. “We will know more after we have made further detailed modelling – and observations, of course. At this time we emphasize the discovery of a
new general relativity phenomenon that allows us to make observations, leaving precise quantitative predictions aside.”
Gravity Probe B - testing the null hypothesis that the spin axis of a gyroscope should stay aligned with a distant reference point when it's in a free fall orbit. But Einstein says no.
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There’s a line out of an early episode of The Big Bang Theory series, where Gravity Probe B is described as having seen ‘glimpses’ of Einstein’s predicted frame-dragging effect. In reality, it is not entirely clear that the experiment was able to definitively distinguish a frame-dragging effect from a background noise created by some exceedingly minor aberrations in its detection system.
Whether or not this counts as a glimpse – frame-dragging (the alleged last untested prediction of general relativity) and Gravity Probe B have become linked in the public consciousness. So here’s a quick primer on what Gravity Probe B may or may not have glimpsed.
The Gravity Probe B satellite was launched in 2004 and set into a 650 kilometer altitude polar orbit around the Earth with four spherical gyroscopes spinning within it. The experimental design proposed that in the absence of space-time curvature or frame-dragging, these gyroscopes moving in a free fall orbit should spin with their axis of rotation unerringly aligned with a distant reference point (in this case, the star IM Pegasi).
To avoid any electromagnetic interference from the Earth’s magnetic field, the gyroscopes were housed within a lead-lined thermos flask – the shell of which was filled with liquid helium. This shielded the instruments from external magnetic interference and the cold enabled superconductance within the detectors designed to monitor the gyroscopes’ spin.
Slowly leaking helium from the flask was also used as a propellant. To ensure the gyroscopes remained in free fall in the event that the satellite encountered any atmospheric drag – the satellite could make minute trajectory adjustments, essentially flying itself around the gyroscopes to ensure they never came in to contact with the sides of their containers.
Now, although the gyroscopes were in free fall – it was a free fall going around and around a space-time warping planet. A gyroscope moving at a constant velocity in fairly empty space is also in a ‘weightless’ free fall – and such a gyroscope could be expected to spin indefinitely about its axis, without that axis ever shifting. Similarly, under Newton’s interpretation of gravity – being a force acting at a distance between massive objects – there is no reason why the spin axis of a gyroscope in a free fall orbit should shift either.
But for a gyroscope moving in Einstein’s interpretation of a steeply curved space-time surrounding a planet, its spin axis should ‘lean over’ into the slope of space-time. So over one full orbit of the Earth, the spin axis will end up pointing in a slightly different direction than the direction it started from – see the animation at the end of this clip. This is called the geodetic effect – and Gravity Probe B did effectively demonstrate this effect’s existence to within only a 0.5% likelihood that the data was showing a null effect.
But, not only is Earth a massive space-time curving object, it also rotates. This rotation should, theoretically, create a drag on the space-time that the Earth is embedded within. So, this frame-dragging should tug something that’s in orbit forward in the direction of the Earth’s rotation.
Where the geodetic effect shifts a polar-orbiting gyroscope’s spin axis in a latitudinal direction – frame-dragging (also known as the Lense-Thirring effect), should shift it in a longitudinal direction.
The expected outcome. Orbiting through warped space-time shifts the spin axis of an gyroscope. But the anticipated frame-dragging shift has proved difficult to detect.
And here is where Gravity Probe B didn’t quite deliver. The geodetic effect was found to shift the gyroscopes spin axis by 6,606 milliarcseconds per year, while the frame-dragging effect was expected to shift it by 41 milliarcseconds per year. This much smaller effect has been difficult to distinguish from a background noise arising from minute imperfections existing within the gyroscopes themselves. Two key problems were apparently a changing polhode path and larger than expected manifestation of a Newtonian gyro torque – or let’s just say that despite best efforts, the gyroscopes still wobbled a bit.
There is ongoing work to laboriously extract the expected data of interest from the noisy data record, via a number of assumptions which might yet be subject to further debate. A 2009 report boldly claimed that the frame-dragging effect is now plainly visible in the processed data – although the likelihood that the data represents a null effect is elsewhere reported at 15%. So maybe glimpsed is a better description for now.
Incidentally, Gravity Probe A was launched back in 1976 – and in a two hour orbit effectively confirmed Einstein’s redshift prediction to within 1.4 parts in 10,000. Or let’s just say that it showed that a clock at 10,000 km altitude was found to run significantly faster than a clock on the ground.
Physicists say they are closer than ever to finding the source of the Universe’s mysterious dark matter, following a better than expected year of research at the Compact Muon Solenoid (CMS) particle detector, part of the Large Hadron Collider (LHC) at CERN in Geneva.
The scientists have now carried out the first full run of experiments that smash protons together at almost the speed of light. When these sub-atomic particles collide at the heart of the CMS detector, the resultant energies and densities are similar to those that were present in the first instants of the Universe, immediately after the Big Bang some 13.7 billion years ago. The unique conditions created by these collisions can lead to the production of new particles that would have existed in those early instants and have since disappeared.
The researchers say they are well on their way to being able to either confirm or rule out one of the primary theories that could solve many of the outstanding questions of particle physics, known as Supersymmetry (SUSY). Many hope it could be a valid extension for the Standard Model of particle physics, which describes the interactions of known subatomic particles with astonishing precision but fails to incorporate general relativity, dark matter and dark energy.
In particle physics, supersymmetry is a symmetry that relates elementary particles of one spin to other particles that differ by half a unit of spin and are known assuperpartners. In a theory with unbroken supersymmetry, for every type of boson there exists a corresponding type of fermion with the same mass and internal quantum numbers, and vice-versa.
Dark matter is an invisible substance that we cannot detect directly but whose presence is inferred from the rotation of galaxies. Physicists believe that it makes up about a quarter of the mass of the Universe whilst the ordinary and visible matter only makes up about 5% of the mass of the Universe. Its composition is a mystery, leading to intriguing possibilities of hitherto undiscovered physics.
Professor Geoff Hall from the Department of Physics at Imperial College London, who works on the CMS experiment, said, “We have made an important step forward in the hunt for dark matter, although no discovery has yet been made. These results have come faster than we expected because the LHC and CMS ran better last year than we dared hope and we are now very optimistic about the prospects of pinning down Supersymmetry in the next few years.”
The energy released in proton-proton collisions in CMS manifests itself as particles that fly away in all directions. Most collisions produce known particles but, on rare occasions, new ones may be produced, including those predicted by SUSY – known as supersymmetric particles, or ‘sparticles’. The lightest sparticle is a natural candidate for dark matter as it is stable and CMS would only ‘see’ these objects through an absence of their signal in the detector, leading to an imbalance of energy and momentum.
In order to search for sparticles, CMS looks for collisions that produce two or more high-energy ‘jets’ (bunches of particles traveling in approximately the same direction) and significant missing energy.
Dr. Oliver Buchmueller, also from the Department of Physics at Imperial College London, but who is based at CERN, said, “We need a good understanding of the ordinary collisions so that we can recognise the unusual ones when they happen. Such collisions are rare but can be produced by known physics. We examined some 3 trillion proton-proton collisions and found 13 ‘SUSY-like’ ones, around the number that we expected. Although no evidence for sparticles was found, this measurement narrows down the area for the search for dark matter significantly.”
The physicists are now looking forward to the 2011 run of the LHC and CMS, which is expected to bring in data that could confirm Supersymmetry as an explanation for dark matter.
The CMS experiment is one of two general purpose experiments designed to collect data from the LHC, along with ATLAS (A Toroidal LHC ApparatuS). Imperial’s High Energy Physics Group has played a major role in the design and construction of CMS and now many of the members are working on the mission to find new particles, including the elusive Higgs boson particle (if it exists), and solve some of the mysteries of nature, such as where mass comes from, why there is no anti-matter in our Universe and whether there are more than three spatial dimensions.
Is it ever possible to find yourself in a situation where you see the hands of a clock freeze? Nnnnnnnnnn….
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There is a story told about traveling at the speed of light in which you are asked to imagine that you begin by standing in front of a big clock – like Big Ben. You realize that your current perception of time is being informed by light reflected off the face of the clock – which is telling you it’s 12:00. So if you then shoot away at the same speed as that light – all you will continue to see is that clock fixed at 12:00, since you are moving at the same speed that this information is moving. And so you discover that at the speed of light, time essentially stands still.
While there are a number of things wrong with this story – as it happens, one correct thing is that if you were able to travel at the speed of light you would experience no passage of time – although there are several reasons why this is probably an impossible situation to find yourself in.
But nonetheless, if you were able to travel at light speed and not experience the passage of time – then you would have no time available to reassess your situation – indeed there would be no time available for your neurons to fire. So, you might well leave Earth with the image of the clock fixed on your retina, but since your brain has stopped working, this has nothing to do with the information carried in the light beam you are moving along with. Your retina is never refreshed with a new image as long as you stay at the speed of light.
Some insight into special relativity is gained by considering the context of someone who stayed back on Earth. If your light speed trip was aimed at a mirror at Alpha Centauri (4.3 light years away) – then from their perspective, it takes you 8.6 years to go there and bounce back. This is true even though you leave and return with an image of 12:00 stuck on your retina and rightly announce that (from your perspective) no time has passed since your departure.
But moving at light speed and experiencing no passage of time is probably an impossible scenario for we mass-challenged beings. Relativity has it that you possess a proper mass, a proper length and a proper time – which persist regardless of your velocity. If you could survive the G forces to get up to such speeds, then you could happily coast at 99.95% of the speed of light and check your pulse against your watch to find your heart still beating at 72 beats per minute – just like it did back on Earth.
It’s only when you check back with Earth that you see that something remarkable is happening. Moving at 99.5% of the speed of light gives you a time dilation factor of around 10. So while someone back on Earth will still measure your trip duration at about 8.6 years – for you it will only be around 10 months. And with a remarkably good telescope you might look back to Earth and see a distorted Big Ben, red-shifted and running slow on the way there and then blue-shifted and running very fast on the way back.
At speeds of less than 10% of the speed of light (0.1c or 30,000 km/sec) time dilation is miniscule, but from 99% speed of light up it increases asymptotically towards infinite.
One of the reasons that probably makes the experience of light speed/time freeze unobtainable is that time dilation keeps increasing the faster you move. For example, at a speed of 99.99995% of the speed of light you get a time dilation factor of about 1,000. So even if you have a spacecraft with an infinite power source capable of seemingly infinite velocities – you will keep arriving at your destination before your speedometer makes it all the way from 99.99999(etc)% of the speed of light to c = 1.0.
This is perhaps how we will populate the universe – using difficult-to-imagine investments of energy, coupled with the principle of time dilation to cross vast distances. The trick is not to get homesick, because after covering such distances you can never go back – unless it is to meet your very, very, very great grandchildren.
(I have cheated a bit by ignoring any periods of acceleration and deceleration within the journeys described here).
Scientists using NASA’s Fermi Gamma-ray Space Telescope have detected beams of antimatter produced above thunderstorms on Earth, a phenomenon never seen before.
Scientists think the antimatter particles were formed in a terrestrial gamma-ray flash (TGF), a brief burst produced inside thunderstorms and shown to be associated with lightning. It is estimated that about 500 TGFs occur daily worldwide, but most go undetected.
“These signals are the first direct evidence that thunderstorms make antimatter particle beams,” said Michael Briggs, a member of Fermi’s Gamma-ray Burst Monitor (GBM) team at the University of Alabama in Huntsville (UAH). He presented the findings Monday, during a news briefing at the American Astronomical Society meeting in Seattle. Continue reading “Fermi Telescope Catches Thunderstorms Hurling Antimatter into Space”
Anyhow, this prompted me to look up different ways in which apparent superluminal motion might be generated, partly to reassure myself that the bottom hadn’t fallen out of relativity physics and partly to see if these things could be adequately explained in plain English. Here goes…
1) Cause and effect illusions
The faster than light pulsar story is essentially about hypothetical light booms – which are a bit like a sonic booms, where it’s not the sonic boom, but the sound source, that exceeds the speed of sound – so that individual sound pulses merge to form a single shock wave moving at the speed of sound.
Now, whether anything like this really happens with light from pulsars remains a point of debate, but one of the model’s proponents has demonstrated the effect in a laboratory – see this Scientific American blog post.
What you do is to arrange a line of light bulbs which are independently triggered. It’s easy enough to make them fire off in sequence – first 1, then 2, then 3 etc – and you can keep reducing the time delay between each one firing until you have a situation where bulb 2 fires off after bulb 1 in less time than light would need to travel the distance between bulbs 1 and 2. It’s just a trick really – there is no causal connection between the bulbs firing – but it looks as though a sequence of actions (first 1, then 2, then 3 etc) moved faster than light across the row of bulbs. This illusion is an example of apparent superluminal motion.
There are a range of possible scenarios as to why a superluminal Mexican wave of synchrotron radiation might emanate from different point sources around a rapidly rotating neutron star within an intense magnetic field. As long as the emanations from these point sources are not causally connected, this outcome does not violate relativity physics.
2) Making light faster than light
You can produce an apparent superluminal motion of light itself by manipulating its wavelength. If we consider a photon as a wave packet, that wave packet can be stretched linearly so that the leading edge of the wave arrives at its destination faster, since it is pushed ahead of the remainder of the wave – meaning that it travels faster than light.
However, the physical nature of ‘the leading edge of a wave packet’ is not clear. The whole wave packet is equivalent to one photon – and the leading edge of the stretched out wave packet cannot carry any significant information. Indeed, by being stretched out and attenuated, it may become indistinguishable from background noise.
Also this trick requires the light to be moving through a refractive medium, not a vacuum. If you are keen on the technical details, you can make phase velocity or group velocity faster than c (the speed of light in a vacuum) – but not signal velocity. In any case, since information (or the photon as a complete unit) is not moving faster than light, relativity physics is not violated.
3) Getting a kick out of gain media
You can mimic more dramatic superluminal motion through a gain medium where the leading edge of a light pulse stimulates the emission of a new pulse at the far end of the gain medium – as though a light pulse hits one end of a Newton’s Cradle and new pulse is projected out from the other end. If you want to see a laboratory set-up, try here. Although light appears to jump the gap superluminally, in fact it’s a new light pulse emerging at the other end – and still just moving at standard light speed.
Light faster than light. Left: Stretching the waveform of light can make the leading edge of the wave seem to move faster than light. Right: Gain media can act like a Newton's Cradle, making light seem to jump the gap superluminally.
4) The relativistic jet illusion
If an active galaxy, like M87, is pushing out a jet of superheated plasma moving at close to the speed of light – and the jet is roughly aligned with your line of sight from Earth – you can be fooled into thinking its contents are moving faster than light.
If that jet is 5,000 light years long, it should take at least 5,000 years for anything in it to cross that distance of 5,000 light years. A photon emitted by a particle of jet material at point A near the start of the jet really will take 5,000 years to reach you. But meanwhile, the particle of jet material continues moving towards you nearly as fast as that photon. So when the particle emits another photon at point B, a point near the tip of the jet – that second photon will reach your eye in much less than 5,000 years after the first photon, from point A. This will give you the impression that the particle crossed 5,000 light years from points A to B in much less than 5,000 years. But it is just an optical illusion – relativity physics remains unsullied.
5) Unknowable superluminal motion
It is entirely possible that objects beyond the horizon of the observable universe are moving away from our position faster than the speed of light – as a consequence of the universe’s cumulative expansion, which makes distant galaxies appear to move away faster than close galaxies. But since light from hypothetical objects beyond the observable horizon will never reach Earth, their existence is unknowable by direct observation from Earth – and does not represent a violation of relativity physics.
And lastly, not so much unknowable as theoretical is the notion of early cosmic inflation, which also involves an expansion of space-time rather than movement within space-time – so no violation there either.
Other stuff…
I’m not sure that the above is an exhaustive list and I have deliberately left out other theoretical proposals such as quantum entanglement and the Alcubierre warp drive. Either of these, if real, would arguably violate relativity physics – so perhaps need to be considered with a higher level of skepticism.
A traditional galaxy evolution model has it that you start with spiral galaxies – which might grow in size through digesting smaller dwarf galaxies – but otherwise retain their spiral form relatively undisturbed. It is only when these galaxies collide with another of similar size that you first get an irregular ‘train-wreck’ form, which eventually settles into a featureless elliptical form – full of stars following random orbital paths rather than moving in the same narrow orbital plane that we see in the flattened galactic disk of a spiral galaxy.
The concept of secular galaxy evolution challenges this notion – where ‘secular’ means separate or isolated. Theories of secular evolution propose that galaxies naturally evolve along the Hubble sequence (from spiral to elliptical), without merging or collisions necessarily driving changes in their form.
While it’s clear that galaxies do collide – and then generate many irregular galaxy forms we can observe – it is conceivable that the shape of an isolated spiral galaxy could evolve towards a more amorphously-shaped elliptical galaxy if they possess a mechanism to transfer angular momentum outwards.
The flattened disk shape of standard spiral galaxy results from spin – presumably acquired during its initial formation. Spin will naturally cause an aggregated mass to adopt a disk shape – much as pizza dough spun in the air will form a disk. Conservation of angular momentum requires that the disk shape will be sustained indefinitely unless the galaxy can somehow lose its spin. This might happen through a collision – or otherwise by transferring mass, and hence angular momentum, outwards. This is analogous to spinning skaters who fling their arms outwards to slow their spin.
Density waves may be significant here. The spiral arms commonly visible in galactic disks are not static structures, but rather density waves which cause a temporary bunching together of orbiting stars. These density waves may be the result of orbital resonances generated amongst the individual stars of the disk.
Left: Density waves may emerge from gravitational resonances generated by the alignment of stars
It has been suggested that a density wave represents a collisionless shock which has a damping effect on the spin of the disk. However, since the disk is only braking upon itself, angular momentum still has to be conserved within this isolated system.
A galactic disk has a corotation radius – a point where stars rotate at the same orbital velocity as the density wave (i.e. a perceived spiral arm) rotate. Within this radius, stars move faster than the density wave – while outside the radius, stars move slower than the density wave.
This may account for the spiral shape of the density wave – as well as offering a mechanism for the outward transfer of angular momentum. Within the radius of corotation, stars are giving up angular momentum to the density wave as they push through it – and hence push the wave forward. Outside the radius of corotation, the density wave is dragging through a field of slower moving stars – giving up angular momentum to them as it does so.
The result is that the outer stars are flung further outwards to regions where they could adopt more random orbits – rather than being forced to conform to the mean orbital plane of the galaxy. In this way, a tightly-bound rapidly spinning spiral galaxy could gradually evolve towards a more amorphous elliptical shape.
Seeking to detect mysterious, ultra-high-energy neutrinos from distant regions of space, a team of astronomers used the Moon as part of an innovative telescope system for the search. Their work gave new insight on the possible origin of the elusive subatomic particles and points the way to opening a new view of the Universe in the future.
The team used special-purpose electronic equipment brought to the National Science Foundation’s Very Large Array (VLA) radio telescope, and took advantage of new, more-sensitive radio receivers installed as part of the Expanded VLA (EVLA) project. Prior to their observations, they tested their system by flying a small, specialized transmitter over the VLA in a helium balloon.
In 200 hours of observations, Ted Jaeger of the University of Iowa and the Naval Research Laboratory, and Robert Mutel and Kenneth Gayley of the University of Iowa did not detect any of the ultra-high-energy neutrinos they sought. This lack of detection placed a new limit on the amount of such particles arriving from space, and cast doubt on some theoretical models for how those neutrinos are produced.
Neutrinos are fast-moving subatomic particles with no electrical charge that readily pass unimpeded through ordinary matter. Though plentiful in the Universe, they are notoriously difficult to detect. Experiments to detect neutrinos from the Sun and supernova explosions have used large volumes of material such as water or chlorine to capture the rare interactions of the particles with ordinary matter.
The ultra-high-energy neutrinos the astronomers sought are postulated to be produced by the energetic, black-hole-powered cores of distant galaxies; massive stellar explosions; annihilation of dark matter; cosmic-ray particles interacting with photons of the Cosmic Microwave Background; tears in the fabric of space-time; and collisions of the ultra-high-energy neutrinos with lower-energy neutrinos left over from the Big Bang.
Radio telescopes can’t detect neutrinos, but the scientists pointed sets of VLA antennas around the edge of the Moon in hopes of seeing brief bursts of radio waves emitted when the neutrinos they sought passed through the Moon and interacted with lunar material. Such interactions, they calculated, should send the radio bursts toward Earth. This technique was first used in 1995 and has been used several times since then, with no detections recorded. The latest VLA observations have been the most sensitive yet done.
“Our observations have set a new upper limit — the lowest yet — for the amount of the type of neutrinos we sought,” Mutel said. “This limit eliminates some models that proposed bursts of these neutrinos coming from the halo of the Milky Way Galaxy,” he added. To test other models, the scientists said, will require observations with more sensitivity.
“Some of the techniques we developed for these observations can be adapted to the next generation of radio telescopes and assist in more-sensitive searches later,” Mutel said. “When we develop the ability to detect these particles, we will open a new window for observing the Universe and advancing our understanding of basic astrophysics,” he said.
The scientists reported their work in the December edition of the journal Astroparticle Physics.
