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NASA Will Be Building a Quiet, Supersonic Aircraft: the X-59

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NASA’s X-Plane Program has been around for 70 years. Over the course of those decades, the agency has developed a series of airplanes and rockets to test out various technologies and design advances. Now NASA has cleared the newest one, the X-59, for final assembly. The X-59 is a supersonic aircraft design. Its full name …

The post NASA Will Be Building a Quiet, Supersonic Aircraft: the X-59 appeared first on Universe Today.



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saganfan
307 days ago
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This is Why Saturn’s Rotation is So Hard to Measure

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For a rocky planet, finding the length of a day can be simple. Just pick a reference point and watch how long it takes to rotate out of view, then back into view. But for planets like Saturn, it’s not so simple. There are no surface features to track. Scientists have spent decades trying to …

The post This is Why Saturn’s Rotation is So Hard to Measure appeared first on Universe Today.

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saganfan
407 days ago
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Telescopes: Refractor vs Reflector

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On the other hand, the refractor's limited light-gathering means it's unable to make out shadow people or the dark god Chernabog.
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saganfan
1363 days ago
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Next time a student asks me about what telescope they should buy I will show them this.
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6 public comments
kizilbash
1359 days ago
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So a refractor can see space vampires?
Arizona
mkalus
1362 days ago
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I like both. My refractor and my reflector.
iPhone: 49.287476,-123.142136
JayM
1362 days ago
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Heh
Atlanta, GA
Atrus
1363 days ago
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Well, there's still a mirror on the refractor too...
Covarr
1363 days ago
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I'm more interested in a redactor, really useful for cleaning up mistakes in a light's output.
Moses Lake, WA
alt_text_bot
1363 days ago
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On the other hand, the refractor's limited light-gathering means it's unable to make out shadow people or the dark god Chernabog.
skittone
1360 days ago
Yay! Alt_text_bot, you're wonderful. Thank you.

Emergent Gravity faces its First Test in Galaxy Lensing

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Title: First test of Verlinde’s theory of Emergent Gravity using Weak Gravitational Lensing measurements

Authors: M. M. Brouwer, M.R. Visser, A Dvornik, et al.

First Author’s Institution: Leiden Observatory, Leiden, The Netherlands

Status: Submitted to The Monthly Notices of the Royal Astronomical Society (MNRAS), December 2016 [open access]


Despite being a near-perfect model and explaining everything ranging from galactic rotation curves to high-redshift supernovae observations, Lambda-CDM has its problems. A lack of clear candidates for a dark matter particle and dark energy are two that certainly keep physicists up at night. This leads us towards alleys unexplored – theories that are creative, innovative and crucial to the scientific process, theories that could lead us to the eventual model of the universe with a clear explanations of all observations. One such theory that garnered some attention in the last few years is Emergent Gravity.

newtheoryofg

Fig 1. Galaxy rotation curves observed over the last few years indicate a dominant matter halo on the outskirts of galaxies, something that’s explained concretely by dark matter.

What is ‘Emergent’ in Emergent Gravity?

The idea is pretty radical yet basic – gravity isn’t a manifestation of mass in spacetime as proposed by Einstein’s General Relativity (GR) or a fundamental force that fits perfectly in a four-force model of the universe. Instead, gravity is proposed to be ’emerging’ from interactions between even more fundamental particles. This is akin to seeing thermodynamical parameters like pressure and temperature arising from interactions between atoms and molecules – what’s crucial to our discussion is the macroscopic quantity. In the case here, that quantity would be gravity. This idea has been developing over the last few decades, with Theodore Jacobson, Thanu Padmanabhan and more recently, Erik Verlinde contributing heavily to its development.

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Fig 2. High speeds of particle collision against the walls of a container lead to higher temperature, since the system possesses more kinetic energy that gets converted to thermal energy.

Diving deep into Entropy and Gravity

One aspect of a theoretical model like emergent gravity (EG) is that we are allowed to derive macroscopic results without having to worry about the underlying fundamental particles that could lead to gravity ’emerging’ – at least for now. This ’emergence’ can be thought of as the result of the tendency of a physical system to increase its entropy. Early work in the field towards a ‘thermodynamics-like theory of gravity’ used something called ‘holographic scaling of entropy’, which essentially scales with surface area of an enclosed volume of spacetime. Verlinde’s new work insists that due to dark energy, we see deviations in GR at long distances that can be resolved if this entropy scaling scales as volume instead of area. Keeping details aside, this leads to a different ‘force-law’, that has additional dominant matter terms that could explain dark matter (called ‘apparent dark matter’ in this case). This and this piece are excellent sources for details on the model. It can be seen that in some sense, this model combines the origin of dark matter and dark energy in a novel way.

Basics of Weak Gravitational Lensing

Well, how do we test this theory? Perhaps, passing it through the same standards as GR would seem appropriate.

The idea of gravitational lensing was one of the first tests of GR i.e. the idea that light’s path gets distorted when traveling through curved spacetime surrounding massive objects. This distortion can change the light ray received from background galaxies (and hence, apparent shape and size) due to a foreground massive object like a galaxy or a galaxy cluster, leading to weak gravitational lensing. This galaxy-galaxy lensing signal is a massive success story of GR, as observations of this phenomena in the Universe fit into the model very well.

hst_lens_smile-jpg-crop-original-original

Fig 3. Gravitational lensing leading to a drastic distortion in light coming from background galaxies. Credit: NASA-Hubble Space Telescope.

Since EG still gives rise to ‘apparent dark matter’, it is safe to say that the gravitational lensing formalism stays the same, since we do apply this formalim to our universe’s dark matter-dominated objects like galaxy clusters (if we believe Lambda-CDM and its predictions). This allows us to use weak lensing as a test for emergent gravity, and match observations against the predictions of this theory.

This work

The regime studied in this work is the low-redshift universe, or the relatively local universe, where the Hubble Constant can be treated as a constant. This is almost true because of the dominance of dark energy after redshift ~0.7-0.9. Since Verlinde’s EG isn’t evolved enough as a theory to quantify cosmology before this epoch, this work assumes a background Lambda-CDM cosmology. For studying galaxy-galaxy lensing, Brouwer et al. select ~33,000 galaxies from the Galaxy And Mass Assembly (GAMA) survey as ‘lenses’ and KiDS survey galaxies as background galaxies that get lensed. They model these galaxies as having a static, spherically symmetric distribution of mass- something like a point mass or an extended source resembling a point mass- because that’s what EG can handle so far.

This work calculates the lensing effect by measuring distortions in the background galaxies’ images, termed as a ‘shear’. In the framework of GR, this quantity is comprised in something called the Extended Surface Density (ESD) profile. Brouwer et al. calculated the ESD for these galaxies under the many assumptions of this model, compared them with Navarror-Frenk-White (NFW) profiles of galaxies from Lambda-CDM, and found that there was general agreement in the ESD progression between the two.

screenshot

Fig 4. From the paper, a model-fit of Emergent Gravity(Point mass model), Emergent Gravity (Extended model) and Dark Matter(NFW model). The lensing signal measured in the form of an ESD is plotted for four different galactic mass bins. It can be seen that Verlinde’s Emergent Gravity model assisted by teh assumptions made by Brouwer et al. match NFW profile predictions very well.

Conclusion and Summary

So what are the assumptions? For one, EG cannot deal with evolution of the universe at the moment. Moreover, the theory isn’t developed enough to have a basic framework of what causes gravity to ’emerge’ from fundamental interactions. The paper agrees that a more ‘sophisticated implementation of both theories’ is needed to make a statement about whether apparent dark matter explains observations better than Lambda-CDM dark matter. Till then, EG shall keep on evolving and observations shall keep on being pitted against these evolving frameworks. A very exciting space to watch!


For a more complete list of assumptions and details of EG in the context of observations, see Section 6, Conclusion of the paper above.

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saganfan
1406 days ago
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Map Age Guide

6 Comments and 20 Shares
Does the screeching chill your blood and herald death? If yes, banshee. If no, seagull.
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saganfan
1602 days ago
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JayM
1599 days ago
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Absolutely awesome. Where did my XKCD subscription disappear to... Grrrr.
Atlanta, GA
darastar
1602 days ago
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Randall has combined many of my nerdy interests into one flow chart: history, geography, fiction.
dukeofwulf
1603 days ago
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It turns out my map is actually a stapler. Long day at the office.
Cthulhux
1603 days ago
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Now this will let me dig deep in Wikipedia for a couple of days. Excuse me, good sirs.
Fledermausland
wafonso
1603 days ago
Now I'm worried about the spiders.
alt_text_bot
1603 days ago
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Does the screeching chill your blood and herald death? If yes, banshee. If no, seagull.

The Milky Way’s hot spot

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The center of our galaxy is a busy place. But it might be one of the best sites to hunt for dark matter.

When you look up at night, the Milky Way appears as a swarm of stars arranged in a misty white band across the sky. 

But from an outside perspective, our galaxy looks more like a disk, with spiral arms of stars reaching out into the universe. At the center of this disk is a small region around which the entire pinwheel of our galaxy rotates, a region packed with exotic astronomical phenomena ranging from dark matter and newborn stars to a supermassive black hole. Astronomers call this region of the Milky Way the galactic center. 

It’s a strange neighborhood, and scientists have reason to believe it’s one of the best places to hunt for dark matter.

The Spitzer Space Telescope provides an infrared view of the galactic center region.

Courtesy of: NASA/JPL-Caltech/ESA/CXC/STScI

Phenomena in our galaxy’s heart

In the ’70s, scientists hypothesized that a supermassive black hole might be lurking in the center of the Milky Way. Black holes are points of space-time where gravity is so strong that not even light can escape.

After decades of trying to indirectly identify the mysterious object in the galactic center by tracing the orbits of stars and gas, astronomers were finally able to calculate its mass in 2008. It weighed more than 4 million times as much as the sun, making it a prime supermassive black hole candidate.

About 10 percent of all new star formation in the galaxy occurs in the galactic center. This is strange because local conditions produce an extreme environment in which it should be difficult for stars to form.

Scientists believe that at least some of the new stars being formed should explode and transform into pulsars, but they aren’t seeing any. Pulsars emit a regular pulsating signal, like a lighthouse. One early explanation for the apparent lack of pulsars in the galactic center was that the magnetic fields there could be bending their radio waves on their way to us, hiding their pulsating signals. But recently scientists measured the strength of the fields and realized the bending was much less than they had anticipated. The mystery of the missing pulsars remains unsolved.

The galactic center also has a notably high concentration of cosmic rays, high-energy charged particles that hurtle through outer space. Scientists still don’t understand where these particles come from or how they reach such intense energies.

The Hubble Space Telescope, though better known for its visible light images, also captured an infrared light picture of the galactic center (the bright patch in the lower right).

Courtesy of: NASA/JPL-Caltech/ESA/CXC/STScI

Hunting for dark matter

We know that the Milky Way is rotating because when we look along it, we see some stars moving towards us and some stars moving away. But the speed at which our galaxy rotates is faster than it should be for the amount of matter we can see. 

This leads scientists to believe that there is matter located in the center of our galaxy that we cannot see. Despite all of the other stuff going on there, this makes the inner galaxy the perfect hunting ground for this “dark matter,” an invisible substance that makes up most of the matter in the universe.

Scientists looking for dark matter take advantage of the fact that it likely interacts with itself. Researchers predict that when dark matter particles run into each other, they annihilate. They believe that this might produce a distinctive spectrum of gamma rays. 

Over the past few years, scientists have detected an excess of gamma rays from the Milky Way’s galactic center. Many scientists believe that this could be a very strong signal for dark matter. The events look the way they would expect dark matter to look, and the energy spectrum and the way the gamma rays are concentrated resemble what scientists would expect from dark matter. 

Other scientists believe that it is pulsars, not dark matter, that create this signal. Because the excess appears clumped, instead of smooth, scientists believe that it could be coming from compact sources like an ancient population of pulsars.

To determine whether this excess is a dark matter signal, scientists are looking for similar signatures elsewhere in the universe, in places like dwarf galaxies. These small galaxies are cleaner places to look for dark matter with a lot less going on, but the trade-off is that they do not produce as much gamma radiation. 

This Chandra X-ray Observatory image distinguishes between lower energy X-rays (pink) and higher energy X-rays (blue).

Courtesy of: NASA/JPL-Caltech/ESA/CXC/STScI

Peering into the galactic center

The galactic center is clouded from our view by about 25,000 light-years of dust and gas, making it difficult to observe in visible light. Scientists have taken to studying different wavelengths, ranging from radio to gamma ray, to tackle the rich landscape of the galactic center. Among the instruments looking at the galactic center, there are a few that see in very different wavelengths.

The W.M. Keck Observatory, a two-telescope observatory near the summit of a dormant volcano in Hawaii, studies the galactic center in the infrared. The Chandra X-ray Observatory, a space observatory launched in 1999, observes the galactic center in X-rays.

Imaging Atmospheric Cherenkov Telescopes are ground-based detectors that scientists use to study gamma rays from the air showers they create when they smash into our atmosphere. The High Energy Stereoscopic System, or HESS, is the world’s largest Cherenkov telescope array and is located in Namibia.

The Fermi Gamma-ray Space Telescope is another observatory scientists use to investigate the galactic center. This is a satellite-based telescope that maps the whole sky at gamma-ray wavelengths. Since its launch in 2008, Fermi has been an important tool in probing the contents of the inner galaxy, from dark matter to pulsars to black holes. 

The longer these instruments collect data, the closer we get to figuring out dark matter and untangling the mess of marvels at the heart of our galaxy.

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saganfan
1666 days ago
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