Would You Notice If An Atom Of Anti-Hydrogen Annihilated In Your Room?

If an atom of anti-hydrogen came into existence in the room you are in, would you notice?
One of the first lead-lead collisions at the Large Hadron Collider, recorded by the ALICE detector in November 2010. In this collision of lead nuclei at a small impact parameter (central collision), 1209 positively-charged (darker tracks) and 1197 negatively-charged (lighter tracks) particles are produced. Image credit:  CERN, for the benefit of the ALICE Collaboration (License: CC-BY-4.0)

One of the first lead-lead collisions at the Large Hadron Collider, recorded by the ALICE detector in November 2010. In this collision of lead nuclei at a small impact parameter (central collision), 1209 positively-charged (darker tracks) and 1197 negatively-charged (lighter tracks) particles are produced. Image credit:  CERN, for the benefit of the ALICE Collaboration (License: CC-BY-4.0)

Anti-hydrogen is the antimatter equivalent of the hydrogen atom. The simplest atom in our Universe, hydrogen is usually made of a single proton and a single electron. Hydrogen is also one of the most abundant elements in our Universe by a large margin, but its antimatter counterpart has been rather difficult...

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How Can We Tell When Something Hits The Moon?

Artist’s conception of the March 17, 2013 lunar impact as seen from Earth. Image credit: NASA's Scientific Visualization Studio

Artist’s conception of the March 17, 2013 lunar impact as seen from Earth. Image credit: NASA's Scientific Visualization Studio

Sound notoriously is poorly transmitted in space; it’s a pressure wave, and there’s just not enough material floating around in space in order for that pressure to survive any distance in space. So anything that happens outside the confines of our little atmosphere is soundless...

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How Dense Is Dark Matter?

Components of the galaxy cluster Abell 2744, also known as the Pandora Cluster: galaxies (white), hot gas (red) and dark matter (blue). The image measures about half a degree across. The image is sprinkled with foreground stars belonging to our Galaxy, the Milky Way, which are visible as the roundish objects with diffraction spikes. Image credit: ESA/XMM-Newton (X-rays); ESO/WFI (optical); NASA/ESA & CFHT (dark matter)

Components of the galaxy cluster Abell 2744, also known as the Pandora Cluster: galaxies (white), hot gas (red) and dark matter (blue). The image measures about half a degree across. The image is sprinkled with foreground stars belonging to our Galaxy, the Milky Way, which are visible as the roundish objects with diffraction spikes. Image credit: ESA/XMM-Newton (X-rays); ESO/WFI (optical); NASA/ESA & CFHT (dark matter)

Originally posted on Forbes!

It very much depends on where you are! Dark matter as we understand it must be some kind of particle, or at least act like some kind of particle. We’re not exactly clear on what the exact nature of that particle would be, or what its individual mass is, or what kind of interactions it ought to have either with itself or with the matter that makes up our planet and all the stars.

But it certainly does seem that dark matter isn’t spread evenly throughout the entire universe. It’s clustered in lumps, and those lumps become the homes to galaxies. Small gatherings of dark matter are generally assumed to be roundish, since that’s the easiest shape for a three dimensional object to form under the influence of gravity.

For galaxy clusters, we can actually map out the shape of the dark matter surrounding these thousands of galaxies by looking at the way that light bends around that part of the Universe. Not all clusters have particularly spherical dark matter surroundings, and we can see the irregularities because the light from galaxies behind the cluster is not bent in the same way along all of the cluster’s edges.

At first glance, this cosmic kaleidoscope of purple, blue and pink offers a strikingly beautiful — and serene — snapshot of the cosmos. However, this multi-coloured haze actually marks the site of two colliding galaxy clusters, forming a single object known as MACS J0416.1-2403 (or MACS J0416 for short). Image credit: NASA, ESA, CXC, NRAO/AUI/NSF, STScI, and G. Ogrean (Stanford University)

At first glance, this cosmic kaleidoscope of purple, blue and pink offers a strikingly beautiful — and serene — snapshot of the cosmos. However, this multi-coloured haze actually marks the site of two colliding galaxy clusters, forming a single object known as MACS J0416.1-2403 (or MACS J0416 for short). Image credit: NASA, ESA, CXC, NRAO/AUI/NSF, STScI, and G. Ogrean (Stanford University)

Within any of these collections of dark matter (technically called halos) surrounding a galaxy or a collection of galaxies, the dark matter is densest at the center, and becomes gradually more diffuse the further out you go. For our own Milky Way, that means that the dark matter density is the highest towards the very center of the galaxy, and out near our solar system, the dark matter density is significantly lower.

Most galaxies contain significantly more mass in dark matter than in luminous matter, but this isn’t because it’s more dense -- the dark matter halo is simply much larger. In the case of the dark matter surrounding our Milky Way, it’s also spherical and not effectively flat, like the bright part of the galaxy is. You can pack a lot more material in a sphere than you can in a circle, so the combination of the dark matter halo being physically larger and a sphere means you wind up with a lot more mass.

This is a mass map of galaxy cluster Cl0024+1654 derived from an extensive Hubble Space Telescope campaign. The colour image is made from two images: a dark-matter map (the blue part of the image) and a 'luminous-matter' map determined from the galaxies in the cluster (the red part of the image). Image credit: European Space Agency, NASA and Jean-Paul Kneib (Observatoire Midi-Pyrénées, France/Caltech, USA)

This is a mass map of galaxy cluster Cl0024+1654 derived from an extensive Hubble Space Telescope campaign. The colour image is made from two images: a dark-matter map (the blue part of the image) and a 'luminous-matter' map determined from the galaxies in the cluster (the red part of the image). Image credit: European Space Agency, NASA and Jean-Paul Kneib (Observatoire Midi-Pyrénées, France/Caltech, USA)

The dark matter density near the solar system, from what I could find, sits at around 0.006 solar masses per cubic parsec, which is a set of units that’s not going to make much sense unless you’re a professional astrophysicist. This is extremely low density. Six-thousandths of a solar mass is approximately the same as six Jupiter mass planets, and a parsec is a 75% of the distance from the Sun to the nearest star. So this means if you wanted to reproduce the dark matter density with the luminous matter that planets are made of, you’d have to clear out a cube of space that’s three light years to a side of absolutely everything. No dust, no gas, no stars, no planets. You get six Jupiters in that box, and you’ll have to spread those Jupiters around, since we don’t have any indication that dark matter comes in chunks.

We can scale this metaphor down a bit; if you wanted to get the same kind of density but in a cubic kilometer, you’d have to evacuate that square kilometer of absolutely every single atom of material. A single grain of birch pollen floating in that cubic kilometer would contain 20 times more mass than there would be in dark matter in that same volume.

At the center of the galaxy, the dark matter should be more than 150 times more concentrated, but this is very difficult to measure within our own galaxy. So far, our observations seem to line up with the models we’ve developed, but there’s definitely room to improve. In any case, 150 times the density of the solar neighborhood is still not very dense! That gets us all of about eight grains of pollen.

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How Big Is The Biggest Galaxy, And How Small Is The Smallest?

This image from the NASA/ESA Hubble Space Telescope shows the rich galaxy cluster Abell 3827. The strange blue structures surrounding the central galaxies are gravitationally lensed views of a much more distant galaxy behind the cluster. Observations of the central four merging galaxies have provided hints that the dark matter around one of the galaxies is not moving with the galaxy itself, possibly implying dark matter-dark matter interactions of an unknown nature are occuring. Image credit: ESO

This image from the NASA/ESA Hubble Space Telescope shows the rich galaxy cluster Abell 3827. The strange blue structures surrounding the central galaxies are gravitationally lensed views of a much more distant galaxy behind the cluster. Observations of the central four merging galaxies have provided hints that the dark matter around one of the galaxies is not moving with the galaxy itself, possibly implying dark matter-dark matter interactions of an unknown nature are occuring. Image credit: ESO

Originally posted on Forbes!

Of all the galaxies in our surveys of the sky, the biggest, most massive systems are always giant elliptical galaxies. These galaxies are thought to be the end product of repeated galactic collisions, and so the end result is an object, which, though full of stars, no longer resembles the galaxy it was, very early in the lifetime of the Universe.

After so many collisions, the stars within these galaxies no longer orbit in an orderly fashion, like those in our own Milky Way, but are more or less moving randomly with respect to each other. Each star follows its own particular elliptical orbit within the gravitational pull of the galaxy, but with no particular regard for what the other stars are doing around it. As a result, the galaxy has no particular directionality to it. If you could hold it in your hands, you could turn it over and around without finding any particular features to give it an orientation.

This NASA/ESA Hubble Space Telescope image shows an elliptical galaxy known as IC 2006. Massive elliptical galaxies like these are common in the modern Universe, but how they quenched their once furious rates of star formation is an astrophysical mystery.  The quenching of star formation seems to have started in the cores of the galaxies and then spread to the outer parts. Image credit: ESA/Hubble & NASA

This NASA/ESA Hubble Space Telescope image shows an elliptical galaxy known as IC 2006. Massive elliptical galaxies like these are common in the modern Universe, but how they quenched their once furious rates of star formation is an astrophysical mystery.  The quenching of star formation seems to have started in the cores of the galaxies and then spread to the outer parts. Image credit: ESA/Hubble & NASA

The very largest of these ellipticals are found in areas of the Universe where many galaxies gather together; with lots of galaxies in the area, there’s more chance for the galaxy to grow into the size it is by devouring its neighbors. The biggest of all are found lurking at the center of galaxy clusters, which are huge associations of thousands of galaxies. The galaxies within the cluster go whirling around the center of mass of those thousands of galaxies, and like the stars within an elliptical, they do this with no particular order.

There’s a little bit of order; the most massive galaxies sink to the center. And so this is where we find the most massive galaxies in our observable Universe. The current heavyweight sits at the center of the cluster Abell 3827 (shown at the top of the page), and has the entirely unpronounceable name of ESO 146-IG 005. It’s in the process of consuming a number of other galaxies, rapidly growing its own mass in the process. This galaxy is currently measured to be 27 trillion times the mass of our sun, which puts it more then ten times the mass of the Milky Way - it is definitely a giant.

Dwarf Galaxy Pisces A. Image credit: NASA, ESA, and E. Tollerud (STScI

Dwarf Galaxy Pisces A. Image credit: NASA, ESA, and E. Tollerud (STScI

The least massive galaxy, on the other hand, is much harder to find, because it's not very bright. By definition, small galaxies have very few stars, so finding that faint light is an observational challenge. These faintest galaxies are extra hard to find because we’re limited to those which are nearby. A faint galaxy too distant from us will be doubly faint, and impossible to spot.

The other problem with defining the least massive galaxy is that the definition of a galaxy gets a little messy at the low mass end. However, if we use the definition that a galaxy has to have some amount of dark matter surrounding it, the current least massive galaxy seems to be Segue 2Segue 2 is about 1000 stars, held together by dark matter, orbiting our Milky Way, and is only about 800 times brighter than our Sun! 

It's worth noting that both of these objects are likely to be holding temporary titles. As observational methods improve, we may find a galaxy was more massive than we had thought, or find dark matter surrounding stars that we had thought had none. In any case, these two objects are the far ends of the galaxy population - from a thousand stars, loosely held together, in the thrall of the Milky Way’s gravitational pull, to something ten times more massive than our entire Milky Way, itself doing the devouring of other galaxies.

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When You Turn Off A Light, Where Does The Light Go?

In space, light will go on, and on, and on… In a windowless room, where does the light go when we switch it off?

Originally posted on Forbes!

Light is a pretty simple beast. In lieu of any interference, it will go on, and on, and on, as we see it doing in the vast, often empty, realms of interstellar and intergalactic space.

Space is a rather unique case, because in between massive objects, light is traveling through something very close to a pure vacuum. This vacuum environment it's traveling through means that there’s very little chance for the light to run into any kind of interference - it's relatively easy for the light to travel enormous distances without anything changing its path or blocking its way.

So what are the options if there is something in the way? Well, light functionally has two options: reflection or absorption. Reflection we’re quite familiar with, as it’s the physics behind seeing yourself in a mirror. This can happen anytime light hits a surface which is smooth, to its perspective. (The smoothness required depends on the wavelength of the light - optical light needs a smoother surface to reflect cleanly off of than radio waves do, which are much longer in wavelength.)

The other option is absorption. This is the process which makes rocks warm in the sun. The rocks are absorbing sunlight over time, and over that time, the energy collected into the rock will warm the surface. Any light can be absorbed, not just the infrared (heat) portion of sunlight. A terrible mirror could absorb enough light that your reflection is only a faint ghost of an image. Unless you’ve got an old filament bulb left in your lamps, you won’t notice an appreciable warming to any of your possessions, because most light bulbs nowadays are designed very specifically not to produce much heat. We can’t see it, and it’s a waste of energy to produce heat which doesn’t help us see the room.

These one-light-year-tall pillars of cold hydrogen and dust, imaged by the Hubble Space Telescope, are located in the Carina Nebula. This image of dust pillars in the Carina Nebula is a composite of 2005 observations taken of the region in hydrogen light (light emitted by hydrogen atoms) along with 2010 observations taken in oxygen light (light emitted by oxygen atoms), both times with Hubble's Advanced Camera for Surveys. The immense Carina Nebula is an estimated 7,500 light-years away in the southern constellation Carina. NASA, ESA, and the Hubble Heritage Project (STScI/AURA); Acknowledgment: M. Livio (STScI) and N. Smith (University of California, Berkeley)

These one-light-year-tall pillars of cold hydrogen and dust, imaged by the Hubble Space Telescope, are located in the Carina Nebula. This image of dust pillars in the Carina Nebula is a composite of 2005 observations taken of the region in hydrogen light (light emitted by hydrogen atoms) along with 2010 observations taken in oxygen light (light emitted by oxygen atoms), both times with Hubble's Advanced Camera for Surveys. The immense Carina Nebula is an estimated 7,500 light-years away in the southern constellation Carina. NASA, ESA, and the Hubble Heritage Project (STScI/AURA); Acknowledgment: M. Livio (STScI) and N. Smith (University of California, Berkeley)

These two options, absorption and reflection, work in tandem with each other, and most materials will do a little of both. Even your standard bathroom mirror absorbs a little of the light that hits it (typically about 10%), and few naturally occurring materials on Earth are perfect light absorbers. Some ultra-black materials are getting very close, but the only truly perfectly absorbing objects around so far are black body objects; an object heated until it glows. (An old filament light bulb would count.)

So, when considering what happens to the light from your light bulb when you switch it off, let’s consider what’s happening when we have the light on. Light is being continually produced by the bulb, which is streaming outwards through the air, mostly unperturbed by having to go through air instead of a vacuum. It will then hit every surface which faces the bulb, and some fraction of it will reflect in the direction of your eyeballs, which will absorb the light, and tell you how bright the room is, along with some information about the objects within the room.

NGC 1999 is an example of a reflection nebula. Like fog around a street lamp, a reflection nebula shines only because the light from an embedded source illuminates its dust; the nebula does not emit any visible light of its own. NGC 1999 lies close to the famous Orion Nebula, about 1,500 light-years from Earth, in a region of our Milky Way galaxy where new stars are being formed actively.  Image Credit: NASA and The Hubble Heritage Team (STScI)

NGC 1999 is an example of a reflection nebula. Like fog around a street lamp, a reflection nebula shines only because the light from an embedded source illuminates its dust; the nebula does not emit any visible light of its own. NGC 1999 lies close to the famous Orion Nebula, about 1,500 light-years from Earth, in a region of our Milky Way galaxy where new stars are being formed actively.  Image Credit: NASA and The Hubble Heritage Team (STScI)

The difference between this situation and switching the light off is simply that you’re no longer replacing the absorbed photons of light with new ones. The last rays of light that the light bulb produced will behave exactly as the rest of the light did: either absorbing into or reflecting off of the various surfaces in your room. The reflected light will bounce until it’s absorbed, but considering how fast the photon can traverse the room, and how few bounces it takes to absorb light, this loss of light is functionally instantaneous.

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