Posted on by Jason Major

A New Spin on Galactic Evolution

Spiral galaxy arms may carry stars along with them, suggests new study

 

There’s a new concept in the works regarding the evolution of galactic arms and how they move across the structure of spiral galaxies. Robert Grand, a postgraduate student at University College London’s Mullard Space Science Laboratory, used new computer modeling to suggest that these signature features of spiral galaxies – including our own Milky Way – evolve in different ways than previously thought.

The currently accepted theory is as spiral galaxies rotate, the “arms” are actually transient structures that move across the flattened disc of stars surrounding the galactic bulge, yet don’t directly affect the movement of the individual stars themselves. This would work in much the same way as a “wave” goes across a crowd at a stadium event. The wave moves, but the individual people do not move along with it – rather, they stay seated after it has passed.

However when Grand researched this suggested motion using computer models of galaxies, he and his colleagues found that this was not what tended to happen. Instead the stars actually moved along with the arms, rather than maintaining their positions.

Also it was observed in these models that the arms themselves are not permanent features, but rather break up and reform over the course of 80 to 100 million years. Grand suggests that this may be due to the powerful gravitational shear forces generated by the spinning of the galaxy.

“We simulated the evolution of spiral arms for a galaxy with five million stars over a period of 6 billion years. We found that stars are able to migrate much more efficiently than anyone previously thought. The stars are trapped and move along the arm by their gravitational influence, but we think that eventually the arm breaks up due to the shear forces.”

– Robert Grand

Snapshots of face-on view of a simulated disc galaxy.

The computer models also showed that the stars along the leading edge of the arms tended to move inwards toward the galactic center while the stars lining the trailing ends were carried to the outer edge of the galaxy.

Since it takes hundreds of millions of years for a spiral galaxy to complete even just one single rotation, observing their evolution and morphology is impossible to do in real time. Researchers like Grand and his simulations are key to our eventual understanding of how these islands of stars formed and continue to shape themselves into the vast, varied structures we see today.

“This research has many potential implications for future observational astronomy, like the European Space Agency’s next corner stone mission, Gaia, which MSSL is also heavily involved in.  As well as helping us understand the evolution of our own galaxy, it may have applications for regions of star formation.”

– Robert Grand

The results were presented at the Royal Astronomical Society’s National Astronomy Meeting in Wales on April 20. Read the press release on the Royal Astronomical Society’s website here.

Top image: M81, a spiral galaxy similar to our own Milky Way, is one of the brightest galaxies that can be seen from Earth. The spiral arms wind all the way down into the nucleus and are made up of young, bluish, hot stars formed in the past few million years, while the central bulge contains older, redder stars. Credit: NASA, ESA, and The Hubble Heritage Team (STScI/AURA)

Posted on by Nancy Atkinson

Early Stars Were Whirling Dervishes

Simulation of the formation of the first stars showing fast rotation. Credit: A. Stacy, University of Texas.

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Even though some of the first stars in the early universe were massive, they probably lived fast and furious lives, as they likely rotated much faster than their present-day counterparts. A new study on stellar evolution looked at a 12-billion-year-old star cluster and found high levels of metal in the stars – a chemical signature that suggests that the first stars were fast spinners.

“We think that the first generations of massive stars were very fast rotators – that’s why we called them spinstars,” said Cristina Chiappini of the Astrophysical Institute Potsdam in Germany, who led the team of astronomers.

These first generation stars died out long ago, and our telescopes can’t look back in time far enough to actually see them, but astronomers can get a glimpse of what they were like by looking at the chemical makeup of later stars. The first stars’ chemical imprints are like fossil records that can be found in the oldest stars we can study.

The general understanding of the early universe is that soon after the Big Bang, the Universe was made of essentially just hydrogen and helium. The chemical enrichment of the Universe with other elements had to wait around 300 million years until the fireworks started with the death of the first generations of massive stars, putting new chemical elements into the primordial gas, which later were incorporated in the next generations of stars.

Using data from ESO’s Very Large Telescope (VLT), the astronomers reanalyzed spectra of a group of very old stars in the Galactic Bulge. These stars are so old that only very massive, short-living stars with masses larger than around ten times the mass of our Sun should have had time to die and to pollute the gas from which these fossil records then formed. As expected, the chemical composition of the observed stars showed elements typical for enrichment by massive stars. However, the new analysis unexpectedly also revealed elements usually thought to be produced only by stars of smaller masses. Fast-rotating massive stars on the other hand would succeed in manufacturing these elements themselves.

“Alternative scenarios cannot yet be discarded – but – we show that if the first generations of massive stars were spinstars, this would offer a very elegant explanation to this puzzle!” said Chiappini.

A star that spins more rapidly can live longer and suffer different fates than slow-spinning ones. Fast rotation also affects other properties of a star, such as its colour, and its luminosity. Spinstars would therefore also have strongly influenced the properties and appearance of the first galaxies which were formed in the Universe. The existence of spinstars is now also supported by recent hydrodynamic simulations of the formation of the first stars of the universe by an independent research group.

Chiappini and her team are currently working on extending the stellar simulations in order to further test their findings. Their work is published in a Nature article on April 28, 2011.

Source: University of Potsdam, Nature

Posted on by Nancy Atkinson

Did the Early Universe Have Just One Dimension?

Planck all-sky image. Credit: ESA, HFI and LFI consortia.

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From a University of Buffalo press release:

Did the early universe have just one spatial dimension? That’s the mind-boggling concept at the heart of a theory that physicist Dejan Stojkovic from the University at Buffalo and colleagues proposed in 2010. They suggested that the early universe — which exploded from a single point and was very, very small at first — was one-dimensional (like a straight line) before expanding to include two dimensions (like a plane) and then three (like the world in which we live today).

The theory, if valid, would address important problems in particle physics.

Now, in a new paper in Physical Review Letters, Stojkovic and Loyola Marymount University physicist Jonas Mureika describe a test that could prove or disprove the “vanishing dimensions” hypothesis.

Because it takes time for light and other waves to travel to Earth, telescopes peering out into space can, essentially, look back into time as they probe the universe’s outer reaches.

Gravitational waves can’t exist in one- or two-dimensional space. So Stojkovic and Mureika have reasoned that the Laser Interferometer Space Antenna (LISA), a planned international gravitational observatory, should not detect any gravitational waves emanating from the lower-dimensional epochs of the early universe.

Stojkovic, an assistant professor of physics, says the theory of evolving dimensions represents a radical shift from the way we think about the cosmos — about how our universe came to be.

The core idea is that the dimensionality of space depends on the size of the space we’re observing, with smaller spaces associated with fewer dimensions. That means that a fourth dimension will open up — if it hasn’t already — as the universe continues to expand.

The theory also suggests that space has fewer dimensions at very high energies of the kind associated with the early, post-big bang universe.

If Stojkovic and his colleagues are right, they will be helping to address fundamental problems with the standard model of particle physics, including the following:

The incompatibility between quantum mechanics and general relativity. Quantum mechanics and general relativity are mathematical frameworks that describe the physics of the universe. Quantum mechanics is good at describing the universe at very small scales, while relativity is good at describing the universe at large scales. Currently, the two theories are considered incompatible; but if the universe, at its smallest levels, had fewer dimensions, mathematical discrepancies between the two frameworks would disappear.

Physicists have observed that the expansion of the universe is speeding up, and they don’t know why. The addition of new dimensions as the universe grows would explain this acceleration. (Stojkovic says a fourth dimension may have already opened at large, cosmological scales.)

The standard model of particle physics predicts the existence of an as yet undiscovered elementary particle called the Higgs boson. For equations in the standard model to accurately describe the observed physics of the real world, however, researchers must artificially adjust the mass of the Higgs boson for interactions between particles that take place at high energies. If space has fewer dimensions at high energies, the need for this kind of “tuning” disappears.

“What we’re proposing here is a shift in paradigm,” Stojkovic said. “Physicists have struggled with the same problems for 10, 20, 30 years, and straight-forward extensions of the existing ideas are unlikely to solve them.”

“We have to take into account the possibility that something is systematically wrong with our ideas,” he continued. “We need something radical and new, and this is something radical and new.”

Because the planned deployment of LISA is still years away, it may be a long time before Stojkovic and his colleagues are able to test their ideas this way.

However, some experimental evidence already points to the possible existence of lower-dimensional space.

Specifically, scientists have observed that the main energy flux of cosmic ray particles with energies exceeding 1 teraelectron volt — the kind of high energy associated with the very early universe — are aligned along a two-dimensional plane.

If high energies do correspond with lower-dimensional space, as the “vanishing dimensions” theory proposes, researchers working with the Large Hadron Collider particle accelerator in Europe should see planar scattering at such energies.

Stojkovic says the observation of such events would be “a very exciting, independent test of our proposed ideas.”

Sources: EurekAlert, Physical Review Letters.

Posted on by Nancy Atkinson

The Universe in a Chocolate Creme Egg

Can chocolate cream eggs help explain the mysteries of the Universe? As part of the University of Nottingham’s Sixty Symbols science video series, the Cadbury creme egg has been featured this week, with several eggcellent videos just in time for Easter. This one discusses the cosmological constant, and the possibility of how we might be surrounded by tiny eggs from another dimension. Surprisingly, scientists can explain and demonstrate the some fundamental scientific laws that govern the universe with yummy cream filled chocolate eggs. See more egg-themed discussions at Sixty Symbols.

Posted on by Ken Kremer

President Obama to Attend Endeavour’s Last Launch on April 29

President Obama plans to attend the last launch of Endeavour on April 29, 2011 at the Kennedy Space Center. President Obama last visited the Kennedy Space Center in Florida on April 15, 2010 and outlined the new course his administration is charting for NASA and the future of U.S. human spaceflight. Credit: NASA/Kim Shiflett

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President Barack Obama and the entire First Family apparently plan to attend the final launch of Space Shuttle Endeavour, according to government officials and multiple news outlets. Endeavour is slated to blast off on the STS-134 mission next Friday, April 29 from the Kennedy Space Center (KSC) in Florida at 3:47 p.m. EDT.

There has already been intense drama surrounding the STS-134 mission because it is being commanded by Mark Kelly. Kelly is the husband of U.S. Congresswoman Gabrielle Giffords of Arizona who was critically wounded by gunshots to her head at point blank range during an assassination attempt while attending a meet and greet with her constituents on Jan. 8, 2011. Six people – including a nine year old girl and a federal judge – were killed and a dozen more were wounded that awful day.

Space Shuttle Endeavour awaits her final launch on April 29, 2011 from Pad 39A at the Kennedy Space Center, FL Credit: Ken Kremer

The Presidents appearance at the STS-134 launch will almost certainly lead to skyrocketing interest, but has not yet been officially announced by NASA and the White House. The event is not yet listed on the presidents official schedule.

However, a tweet by the staff of Congresswoman Giffords on her official website states Obama will attend; “We are very happy that Pres. Obama is coming to Mark’s launch! This historic mission will be #Endeavours final flight.”

NASA spokesman Allard Beutel told me today, “I cannot confirm whether the president will be coming to launch next week. If he’s coming, which I can’t confirm, we are a White House agency.”

“We always welcome a visit from the President,” Beutel said.

Security is always tight at KSC during a shuttle launch. A visit by President Obama will certainly lead to even tighter security controls and even more massive traffic jams.

Giant crowds were already expected for this historic final spaceflight of Space Shuttle Endeavour, NASA’s youngest Orbiter, on her 25th mission to space.

Endeavour is carrying the $2 Billion Alpha Magnetic Spectrometer (AMS) ) on a 14-day flight to the International Space Station, a premier science instrument that will collect cosmic rays, search for dark energy, dark matter and anti matter and seeks to determine the origin of the Universe. See my photo below of the AMS from inside the Space Station Processing Facility (SSPF) at KSC with the principal investigator, Nobel Prize winner Prof. Sam Ting of MIT.

NASA Administrator Charles Bolden just announced that Endeavour will be displayed at the California Science Museum following her retirement from active flight service upon landing.

President Obama last visited KSC on April 15, 2010 and gave a major policy speech outlining his radical new human spaceflight goals for NASA. Obama decided to cancel NASA’s Project Constellation ‘Return to the Moon’ Program and the Ares 1 and Ares 5 rockets. He directed NASA to plan a mission for astronauts to visit an Asteroid by 2025 and one of the moons of Mars in the 2030’s. Obama also decided to revive the Orion crew module built by Lockheed Martin, which is now envisaged for missions beyond low earth orbit (LEO), and invest in development of new commercial space taxis such as the Dragon spacecraft by SpaceX for transporting astronaut crews to the ISS.

Spokesman Beutel said that during the April 2010 visit, “The President met with space workers.” He could not comment on details of the president’s plans for the STS-134 visit and said information would have to come from the White House.

The last time a sitting president watched a live human space launch was in 1998 when then President Bill Clinton attended the blastoff of the return to space of Astronaut and Senator John Glenn. Glenn was the first American to orbit the Earth back in 1962. Glenn’s first flight took place a little over a year after the historic first human spaceflight by Soviet Cosmonaut Yuri Gagarin on April 12, 1961- which occurred exactly 50 years ago last week.

Congresswoman Giffords is recovering from her wounds and Shuttle Commander Kelly has said that she would like to attend the STS-134 launch. But no official announcement about her attendance has been made by NASA and depends on many factors including decisions by the doctors treating her in a Houston area hospital.

The Alpha Magnetic Spectrometer (AMS) and Nobel Prize Winner and Principal Investigator Sam Ting of MIT - inside the Space Station Processing Facility at KSC. The STS-134 mission of shuttle Endeavour will deliver the AMS to the ISS. The AMS purpose is to try and determine the origin of the Universe. . Credit: Ken Kremer
Close up of Endeavour crew cabin, ET, SRB and astronaut walkway to the White Room. Credit: Ken Kremer
Posted on by Vanessa Janek

Antigravity Could Replace Dark Energy as Cause of Universe’s Expansion

Annihilation
Illustration of Antimatter/Matter Annihilation. (NASA/CXC/M. Weiss)

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Since the late 20th century, astronomers have been aware of data that suggest the universe is not only expanding, but expanding at an accelerating rate. According to the currently accepted model, this accelerated expansion is due to dark energy, a mysterious repulsive force that makes up about 73% of the energy density of the universe. Now, a new study reveals an alternative theory: that the expansion of the universe is actually due to the relationship between matter and antimatter. According to this study, matter and antimatter gravitationally repel each other and create a kind of “antigravity” that could do away with the need for dark energy in the universe.

Massimo Villata, a scientist from the Observatory of Turin in Italy, began the study with two major assumptions. First, he posited that both matter and antimatter have positive mass and energy density. Traditionally, the gravitational influence of a particle is determined solely by its mass. A positive mass value indicates that the particle will attract other particles gravitationally. Under Villata’s assumption, this applies to antiparticles as well. So under the influence of gravity, particles attract other particles and antiparticles attract other antiparticles. But what kind of force occurs between particles and antiparticles?

To resolve this question, Villata needed to institute the second assumption – that general relativity is CPT invariant. This means that the laws governing an ordinary matter particle in an ordinary field in spacetime can be applied equally well to scenarios in which charge (electric charge and internal quantum numbers), parity (spatial coordinates) and time are reversed, as they are for antimatter. When you reverse the equations of general relativity in charge, parity and time for either the particle or the field the particle is traveling in, the result is a change of sign in the gravity term, making it negative instead of positive and implying so-called antigravity between the two.

Villata cited the quaint example of an apple falling on Isaac Newton’s head. If an anti-apple falls on an anti-Earth, the two will attract and the anti-apple will hit anti-Newton on the head; however, an anti-apple cannot “fall” on regular old Earth, which is made of regular old matter. Instead, the anti-apple will fly away from Earth because of gravity’s change in sign. In other words, if general relativity is, in fact, CPT invariant, antigravity would cause particles and antiparticles to mutually repel. On a much larger scale, Villata claims that the universe is expanding because of this powerful repulsion between matter and antimatter.

What about the fact that matter and antimatter are known to annihilate each other? Villata resolved this paradox by placing antimatter far away from matter, in the enormous voids between galaxy clusters. These voids are believed to have stemmed from tiny negative fluctuations in the primordial density field and do seem to possess a kind of antigravity, repelling all matter away from them. Of course, the reason astronomers don’t actually observe any antimatter in the voids is still up in the air. In Villata’s words, “There is more than one possible answer, which will be investigated elsewhere.” The research appears in this month’s edition of Europhysics Letters.

Posted on by Steve Nerlich

Astronomy Without A Telescope – Assumptions

This model assumes the cosmological principle. The LCDM universe is homogeneous and isotropic. Time dilation and redshift z are attributed to a Doppler-like shift in electromagnetic radiation as it travels across expanding space. This model assumes a nearly "flat" spatial geometry. Light traveling in this expanding model moves along null geodesics. Light waves are 'stretched' by the expansion of space as a function of time. The expansion is accelerating due to a vacuum energy or dark energy inherent in empty space. Approximately 73% of the energy density of the present universe is estimated to be dark energy. In addition, a dark matter component is currently estimated to constitute about 23% of the mass-energy density of the universe. The 5% remainder comprises all the matter and energy observed as subatomic particles, chemical elements and electromagnetic radiation; the material of which gas, dust, rocks, planets, stars, galaxies, etc., are made. This model includes a single originating big bang event, or initial singularity, which constitutes an abrupt appearance of expanding space containing radiation. This event was immediately followed by an exponential expansion of space (inflation).

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The current standard model of the universe, Lambda-Cold Dark Matter, assumes that the universe is expanding in accordance with the geometrical term Lambda – which represents the cosmological constant used in Einstein’s general relativity. Lambda might be assumed to represent dark energy, a mysterious force driving what we now know to be an accelerating expansion of space-time. Cold dark matter is then assumed to be the scaffolding that underlies the distribution of visible matter at a large scale across the universe.

But to make any reasonable attempt at modelling how the universe is – and how it unfolded in the past and will unfold in the future – we first have to assume that it is roughly the same everywhere.

This is sometimes called the Cosmological Principle which states that when viewed on a sufficiently large scale, the properties of the Universe are the same for all observers. This captures two concepts – that of isotropy, which means that the universe looks roughly the same anywhere you (that is you) look – and homogeneity, which means the properties of the universe look roughly the same for any observers anywhere they are and wherever they look. Homogeneity is not something we can expect to ever confirm by observation – so we must assume that the part of the universe we can directly observe is a fair and representative sample of the rest of the universe.

An assessment of isotropy is at least theoretically possible down our past light-cone. In other words, we look out into the universe and receive historical information about how it behaved in the past. We then assume that those parts of the universe we can observe have continued to behave in a consistent and predictable manner up until the present – even though we can’t confirm whether this is true until more time has passed. But anything outside our light cone is not something we can expect to ever know about and hence we can only ever assume the universe is homogenous throughout.

You occupy a position in space-time from which a proportion of the universe can be observed in your past light cone. You can also shine a torch beam forwards towards a proportion of the future universe - knowing that one day that light beam can reach an object that lies in your future light cone. However, you can never know about anything happening right now at a distant position in space - because it lies on the 'hypersurface of the present'. Credit: Aainsqatsi.

Maartens has a go a developing at developing an argument as to why it might be reasonable for us to assume that the universe is homogenous. Essentially, if the universe we can observe shows a consistent level of isotropy over time, this strongly suggests that our bit of the universe has unfolded in a manner consistent with it being a part of a homogenous universe.

The isotropy of the observable universe can be strongly implied if you look out in any direction and find:
• consistent matter distribution;
• consistent bulk velocities of galaxies and galactic clusters moving away from us via universal expansion.
• consistent measurements of angular diameter distance (where objects of the same absolute size look smaller at a greater distance – until a distance of redshift 1.5, when they start looking larger – see here); and
• consistent gravitational lensing by large scale objects like galactic clusters.

These observations support the assumption that both matter distribution and the underlying space-time geometry of the observable universe is isotropic. If this isotropy is true for all observers then the universe is consistent with the Friedmann–Lemaître–Robertson–Walker (FLRW) metric. This would mean it is homogenous, isotropic and connected – so you can travel anywhere (simply connected) – or it might have wormholes (multiply connected) so not only can you travel anywhere, but there are short cuts.

That the observable universe has always been isotropic – and is likely to continue being so into the future – is strongly supported by observations of the cosmic microwave background, which is isotropic down to a fine scale. If this same isotropy is visible to all observers – then it is likely that the universe has, is and will always be homogenous as well.

Finally, Maartens appeals to the Copernican Principle – which says that not only are we not the center of the universe, but our position is largely arbitrary. In other words, the part of the universe we can observe may well be a fair and representative sample of the wider universe.

Further reading: Maartens Is the universe homogenous?

Posted on by Nancy Atkinson

A New Way to Visualize Warped Space and Time

By combining theory with computer simulations, Thorne and his colleagues at Two doughnut-shaped vortexes ejected by a pulsating black hole. Also shown at the center are two red and two blue vortex lines attached to the hole, which will be ejected as a third doughnut-shaped vortex in the next pulsation. Credit: The Caltech/Cornell SXS Collaboration

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Trying to understand the warping of space and time is something like visualizing a scene from Alice in Wonderland where rooms can change sizes and locations. The most-used description of the warping of space-time is how a heavy object deforms a stretched elastic sheet. But in actuality, physicists say this warping is so complicated that they really haven’t been able to understand the details of what goes on. But new conceptual tools that combines theory and computer simulations are providing a better way to for scientists to visualize what takes place when gravity from an object or event changes the fabric of space.

Researchers at Caltech, Cornell University, and the National Institute for Theoretical Physics in South Africa developed conceptual tools that they call tendex lines and vortex lines which represent gravitation waves. The researchers say that tendex and vortex lines describe the gravitational forces caused by warped space-time and are analogous to the electric and magnetic field lines that describe electric and magnetic forces.

“Tendex lines describe the stretching force that warped space-time exerts on everything it encounters,” said says David Nichols, a Caltech graduate student who came up with the term ‘tendex.’. “Tendex lines sticking out of the Moon raise the tides on the Earth’s oceans, and the stretching force of these lines would rip apart an astronaut who falls into a black hole.”

Vortex lines, on the other hand, describe the twisting of space. So, if an astronaut’s body is aligned with a vortex line, it would get wrung like a wet towel.

Two spiral-shaped vortexes (yellow) of whirling space sticking out of a black hole, and the vortex lines (red curves) that form the vortexes. Credit: The Caltech/Cornell SXS Collaboration

They tried out the tools specifically on computer simulated black hole collisions, and saw that such impacts would produce doughnut-shaped vortex lines that fly away from the merged black hole like smoke rings. The researchers also found that a bundle of vortex lines spiral out of the black hole like water from a rotating sprinkler. Depending on the angles and speeds of the collisions, the vortex and tendex lines — or gravitational waves — would behave differently.

“Though we’ve developed these tools for black-hole collisions, they can be applied wherever space-time is warped,” says Dr. Geoffrey Lovelace, a member of the team from Cornell. “For instance, I expect that people will apply vortex and tendex lines to cosmology, to black holes ripping stars apart, and to the singularities that live inside black holes. They’ll become standard tools throughout general relativity.”

The researchers say the tendex and vortex lines provide a powerful new way to understand the nature of the universe. “Using these tools, we can now make much better sense of the tremendous amount of data that’s produced in our computer simulations,” says Dr. Mark Scheel, a senior researcher at Caltech and leader of the team’s simulation work.

Their paper has been published in the April 11 in the Physical Review Letters.

Source: CalTech

Posted on by Jon Voisey

Halos Gone MAD

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

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One of the successes of the ΛCDM model of the universe is the ability for models to create structures of with scales and distributions similar to those we view in the universe today. Or, at least that’s what astronomers tell us. While computer simulations can recreate numerical universes in a box, interpreting these mathematical approximations is a challenge in and of itself. To identify the components of the simulated space, astronomers have had to develop tools to search for structure. The results has been nearly 30 independent computer programs since 1974. Each promises to reveal the forming structure in the universe by finding regions in which dark matter halos form. To test these algorithms out, a conference was arranged in Madrid, Spain during the May of 2010 entitled “Haloes going MAD” in which 18 of these codes were put to the test to see how well they stacked up.

Numerical simulations for universes, like the famous Millennium Simulation begin with nothing more than “particles”. While these were undoubtedly small on a cosmological scale, such particles represent blobs of dark matter with millions or billions solar masses. As time is run forwards, they are allowed to interact with one another following rules that coincident with our best understanding of physics and the nature of such matter. This leads to an evolving universe from which astronomers must use the complicated codes to locate the conglomerations of dark matter inside which galaxies would form.

One of the main methods such programs use is to search for small overdensities and then grow a spherical shell around it until the density falls off to a negligible factor. Most will then prune the particles within the volume that are not gravitationally bound to make sure that the detection mechanism didn’t just seize on a brief, transient clustering that will fall apart in time. Other techniques involve searching other phase spaces for particles with similar velocities all nearby (a sign that they have become bound).

To compare how each of the algorithms fared, they were put through two tests. The first, involved a series of intentionally created dark matter halos with embedded sub-halos. Since the particle distribution was intentionally placed, the output from the programs should correctly find the center and size of the halos. The second test was a full fledged universe simulation. In this, the actual distribution wouldn’t be known, but the sheer size would allow different programs to be compared on the same data set to see how similarly they interpreted a common source.

In both tests, all the finders generally performed well. In the first test, there were some discrepancies based on how different programs defined the location of the halos. Some defined it as the peak in density, while others defined it as a center of mass. When searching for sub-halos, ones that used the phase space approach seemed to be able to more reliably detect smaller formations, yet did not always detect which particles in the clump were actually bound. For the full simulation, all algorithms agreed exceptionally well. Due to the nature of the simulation, small scales weren’t well represented so the understanding of how each detect these structures was limited.

The combination of these tests did not favor one particular algorithm or method over any other. It revealed that each generally functions well with regard to one another. The ability for so many independent codes, with independent methods means that the findings are extremely robust. The knowledge they pass on about how our understanding of the universe evolves allows astronomers to make fundamental comparisons to the observable universe in order to test the such models and theories.

The results of this test have been compiled into a paper that is slated for publication in an upcoming issue of the Monthly Notices of the Royal Astronomical Society.

Posted on by Steve Nerlich

Astronomy Without A Telescope – Our Inferred Universe

A galaxy far, far away - long. long ago. UDFy-38135539 - the confirmed most distant observed object, where UDF stands for (Hubble) Ultra-Deep Field.

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The universe is a big place – and getting bigger all the time – so at a large scale all unbound structures are all moving away from each other. So when we look out at distant objects, we need to remind ourselves that not only are we seeing them as they appeared in the past, when the light that hits our eyes first left them, but also that they are no longer in that location where they appear to be.

This issue reaches an extreme when we consider observations of the first luminous stars and galaxies – with the galaxy UDFy-38135539 currently holding the record as the most distant object observed and one of the youngest, existing 13.1 billion years ago – although UDFj-39546284 may be the next contender at 13.2 billion years old, subject to further spectroscopic confirmation.

UDFy-38135539 has a redshift (z) of 10 and provides no measurable light at visible wavelengths. Although light from it took 13.1 billion years ago to reach us – it is not correct to say that it is 13.1 billion light years away. In that intervening period, both it and us have moved further away from each other.

So not only is it now further away than it appears, but when the light that we see now was first emitted, it and the location that we now occupy were much closer together than 13.1 billion light years. For this reason it appears larger, but much dimmer than it would appear in a static universe – where it might genuinely be 13.1 billion light years away.

So we need to clarify UDFy-38135539’s distance as a comoving distance (calculated from its apparent distance and the assumed expansion rate of the universe). This calculation would represent the proper distance between us and it – as if a tape measure could be right now instantaneously laid down between us and it.

This distance works out to be about 30 billion light years. But we are just guessing that UDFy-38135539 is still there – more likely it has merged with other young galaxies – perhaps becoming part of a huge spiral galaxy similar to our own Milky Way, which itself contains stars that are over 13 billion years old.

The observable - or inferred - universe. Even this may just be a small component of the whole ball game. At this scale, our immediate galactic neighborhood, the Virgo Supercluster, is too small to be seen. And it is extremely unlikely that it represents the center of the universe. Credit: Azcolvin429.

It is generally said that the comoving distance to the particles that emitted the cosmic microwave background is about 45.7 billion light years away – even though the photons those particles emitted have only been traveling for almost 13.7 billion years. Similarly, by inference, the absolute edge of the observable universe is 46.6 billion light years away.

However, you can’t conclude that this is the actual size of the universe – nor should you conclude that the cosmic microwave background has a distant origin. Your coffee cup may contain particles that originally emitted the cosmic microwave background – and the photons they emitted may be 45.7 billion light years away now – perhaps just now being collected by alien astronomers who will hence have their own 46.6 billion light year radius universe to infer – most of which they can’t directly observe either.

All universal residents have to infer the scale of the universe from the age of the photons that come to us and the other information that they carry. And this will always be historical information.

From Earth we can’t expect to ever come to know about anything that is happening right now in objects that are more distant than a comoving distance of around 16 billion light years, being the cosmic event horizon (equivalent to a redshift of around z = 1.8).

This is because those objects are right now receding from us at faster than the speed of light, even though we may continue receiving updated historical data about them for many billion of years to come – until they become so redshifted as to appear to wink out of existence.

Further reading: Davis and Lineweaver. Expanding Confusion: common misconceptions of cosmological horizons and the superluminal expansion of the universe.