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How a new map of the early universe is like a hedgehog


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'Like painting a map on the back of a hedgehog.'

'So we kind of have this map painted on the back of a hedgehog, and we're resconstrucing what we think is a true 3D map of the universe.' Photo credit: Denis Defreyne

A new method for investigating dark energy has allowed astronomers to peer further back into the past than ever before, revealing a universe that was very different to the one we live in today.

Today’s universe is expanding. Not just that, its expansion is accelerating. Galaxies are not only getting further away from us all of the time, but they’re doing so faster and faster with each passing day.

It has been this way for as long as we’ve been around and a long, long time before then too. We didn’t always know it, though. In fact, until 1998 we thought the expansion was slowing down. But since the universe was around six billion years old, it has been dominated by an unknown force we call dark energy that is pushing everything further and further apart.

Dark energy is weird. As Katie Mack said on her blog earlier this year:

It’s difficult to express to a non-physicist just how weird dark energy is, because most people are used to encountering things that they don’t understand in physics, and they generally assume that someone else is on top of the situation. That’s not the case with dark energy.

We’re pretty sure there was a time before dark energy. In the first six billions years or so after the universe came into existence, its expansion was controlled by matter. This meant that the expansion of the universe actually slowed down with time, because gravity brought everything together.

But today dark energy acts like a kind of anti-gravity, pushing galaxies apart faster than gravity can pull them together.

To try to work out what dark energy actually is, some astronomers make maps. By measuring distances in the universe, they can work out exactly how fast the universe is expanding and measure the effects of dark energy. Measuring what dark energy does can help us to narrow down what it is.

So far, all of these maps have been of times after dark energy started to take over. One reason for that is that it is difficult to study the early universe because there aren’t many bright objects, says David Schelgel, principal investigator on an experiment called BOSS, the Baryon Oscillation Spectroscopic Survey.

So, to get a glimpse of that early period, what BOSS has now done (and detailed in a paper the collaboration published on arXiv today) is look at some of the brightest objects in the early universe: quasars.

Quasars are active galaxies (the name is a contraction of “quasi-stellar radio source”) that exist in the early universe. And they’re bright. Really bright. Schlegel and the BOSS team use the quasars as “backlights” to the hydrogen gas in between us and them.

They have made a map of the line of sight to each quasar. “It’s like skewers through the universe,” he says. From the skewers they reconstruct what they think is a true 3D map of the universe. “One of the cutest descriptions of this is that it’s like you paint a map on the back of a porcupine.”

In these skewers of light astronomers see markers, or absorption lines, from when the light passed through an intervening hydrogen gas cloud on its way from the quasar. Often the light will pass through hundreds of hydrogen clouds. The markers reveal the distribution of the gas along the line of sight to each quasar, and then the astronomers look at all of the lines and use them to work out the whole map just from the skewers.

To do this, you need enough quasars. The BOSS team observed 48,129 quasars to build their 3D map. The quasars they’ve studied reach as far back as 11.5 billion years, into the matter dominated era. In other words, before dark energy took over.

“It’s the first measurement when the universe was slowing down,” says Schlegel. “Which is pretty neat.”

In this map they looked for a structure known as baryon acoustic oscillations, which are essentially sound waves frozen into the early universe. Astronomers know how big these sound waves were when they formed. By measuring them later on in cosmic history they can tell how much the universe has expanded in the mean time. Because the expansion is governed by dark energy, measuring baryon acoustic oscillations provides a way to test whether dark energy is behaving as we think it should.

The BOSS team saw the pattern of deceleration of expansion, before dark energy kicked in, and later acceleration that they expected.

But this study only mapped a third of the volume that BOSS will eventually map. In the next few years BOSS will map a million and a half galaxies and 160,000 quasars.

Watch this space.

Update 03/12/12: Peter Edmonds points out that some work has in fact measured the time when cosmic deceleration turned into acceleration before, using supernovae rather than BAOs. Read more on his blog.

Kelly Oakes About the Author: Kelly Oakes has a master's in science communication and a physics degree, both from Imperial College London. Now she spends her days writing about science. Follow on Twitter @kahoakes.

The views expressed are those of the author and are not necessarily those of Scientific American.





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  1. 1. rloldershaw 10:45 pm 11/13/2012

    Several theorists have written papers on the interpretation of “dark energy” and cosmic acceleration as simply resulting from gravitational backreaction in an inhomogeneous cosmos.

    Perhaps our deeply ingrained faith in homogeneous cosmological models prevents us from seeing the simplicity and beauty of this and related approaches to understanding periods of cosmological acceleration.

    Personally, as one who thinks that it is virtually self-evident that nature has a discrete fractal organization, I am fairly sure that sooner or later we will move to an inhomogeneous cosmological model that is only statistically “homogeneous” over limited scale ranges.

    Robert L. Oldershaw
    Discrete Fractal Cosmology
    http://www3.amherst.edu/~rloldershaw

    Link to this
  2. 2. jtdwyer 6:05 am 11/14/2012

    “… In the first six billions years or so after the universe came into existence, its expansion was controlled by matter. This meant that the expansion of the universe actually slowed down with time, because gravity brought everything together.”

    I don’t follow the reasoning behind the idea that the earlier universe was dominated by matter, thus expansion was decelerating. It seem to me that if matter (gravitation) had dominated universal development their would have been no expansion. That the universe initially expanded requires some energy that dominated any gravitational effects that might have opposed expansion.

    Moreover, since the increasing distances produced by expansion must have diminished any universal effects of gravitation, I don’t see how diminishing gravitational effects would have been slowing expansion.

    As I understand, universal spacetime expansions would be the result of some vacuum energy contained with spacetime. Expansion would have been diminishing because that energy was being dispersed by expansion. Expansion would have been decelerating as a function of the second law of thermodynamics, not as a result of any effects of gravitation that had already been overcome and had also been diminishing.

    Any contraction of spacetime produced by gravitation is localized, diminishing with distance. The universal effect of gravitation is to locally condense matter.

    As galaxies have continuously been merging and becoming increasingly bound within ever larger clusters, universal matter has become increasingly localized as universal spacetime has been expanding.

    The result has been the formation of the ‘cosmic web’ – strands of matter surrounded by continuously growing voids – enormous regions effectively devoid of matter. As these growing regions of expanding spacetime are increasingly free of any gravitational effects that might be in opposition to expansion, their rate of expansion should accelerate.

    In this view, the earlier expanding universe was full of disperse matter that constrained the rate of expansion. While dispersal of universal energy worked to diminish the rate of expansion over time, the increasing domination of growing voids within the universe has worked to increase the universal rate of expansion.

    While expansion has always been produced by (diminishing) vacuum energy density, the diminishing constraints of increasingly localized gravitational matter have produced a universe increasingly dominated by voided regions containing expansion producing vacuum energy. While expansion energy is continuously diminished by dispersion, the increasing localization of universal gravitation allows increasing rates of expansion.

    Link to this
  3. 3. vinodkumarsehgal 8:20 am 11/14/2012

    Observed red shift of light emanated billion of years in past from remote galaxies has been interpreted by Scientists to arise from expansion of universe ( or space). Except this, there has been no explanation for the physical mechanism for the expansion of space. Expansion of space has been interpreted to cause drag of physical galaxies along with it, however, there has been complete lack of understanding of any interacting force between material galaxies and space. Physicality of space is not understood well. It is not even known if there is any physicality of space at all. However vacuum energy or some expansion energy has been theorized to account for expansion of space and now dark energy has been added to the list of theorized entities to account for the accelerated expansion of space. Physicists have devised a nos. of theorized entities to account for the observation of certain phenomena, however, they lack any understanding of the physical mechanism behind those entities. Above are some of the apparent contradictions in the whole concept of expansion of space — normal or accelerated one.

    There has been no direct measurement of expansion of space. Expansion of space has been hypothesized as a theoretical construct to explain the red shift of remote galaxies. No cosmological shift has also been observed for galaxies positioned within a distance of about 1 billion light years away from earth. But there has been no explanation for lack of cosmological red shift from galaxies within this distance.

    Lack of any model for expansion of space based upon physical mechanism is one of the weakest link in the entire concept of expansion of space.

    A large disparity has been observed in theoretical and observed values of vacuum energy. There has also been no convincing explanation as to how some energy ( dark energy) appeared on the scene suddenly from somewhere some 6 billion years ago to cause accelerated expansion of space.

    Link to this
  4. 4. vinodkumarsehgal 8:53 am 11/14/2012

    It is yet not known fully if theorized vacuum energy or universal energy, as some people call it, is intrinsic or extrinsic to the very existence of space. If vacuum energy is very much ingrained in the existence of space itself I intrinsic) it will imply that creation of more and more space, resulting from expansion, will also bring additional vacuum energy along with it. In view of this, there should be no question of diminishing density of vacuum energy. Density of Vacuum energy or expansion energy should remain constant in all areas of space all the the times in the past, present and future.

    However, if universal or vacuum energy is extrinsic to space this will imply that at the time of Big Bang, there was constant reservoir of some universal energy. in this case, density of vacuum energy should decrease with expanding space.

    Gravitational force manifest thru action on material bodies as evident from increasing or decreasing distance between such bodies. Universal or vacuum energy manifest thru action on space due to increasing expansion. There should be no connection between gravitational energy and universal energy as the entities on which they act are different and nature of their action has also been different. No interaction or relation has been understood between space and matter. Despite all this, some people and even learned scientists come out too often theoretical models based upon vacuum energy ( or universal energy) and gravitation energy to account for the sudden accelerated expansion some 6 billion years ago

    Link to this
  5. 5. jtdwyer 9:04 am 11/14/2012

    In my preceding comment I provided proposed physical mechanism for the the acceleration of universal expansion, as a necessary consequence of spacetime expansion and gravitational localization, that does not require the ‘appearance’ of some new dark energy 7.5 billion years ago (as the article states, expansion is thought to have been diminishing during the first 6 billion years).

    If the space separating galaxies is generally expanding (excluding galaxies that are effectively gravitationally bound to sufficiently proximal neighbors), there does not need to be any ‘dragging’ of galaxies – the expansion of spacetime necessarily increases separation distance without applying any energy to move galaxies.

    The vacuum energy I propose to be responsible for producing the expansion of spacetime in the absence of matter can also provide a physical mechanism producing the localizing gravitational effects described by general relativity as geometric effects using an abstract system of dimensional coordinates.

    I suggest that the geometry of spacetime expressed as dimensional coordinates cannot physically produce the accelerating effects of gravitation. I propose instead that the geometry of spacetime accurately represents localized gradients of vacuum energy density that physically produce gravitation’s accelerating effects.

    While the accretion of matter is generally thought to be the product of some material self-attraction, I suggest that gravitation is not a property of matter at all, but rather a property of spacetime. As vacuum energy accelerates matter (inversely proportionate to its mass-energy property), its momentum as it is extracted from spacetime redirects vacuum energy. The result is the increasing local concentration of matter and the local contraction of vacuum energy density.

    The gravitational acceleration of material objects is then not the product of localized objects of mass, but external gradients of vacuum energy, locally condensed by the accretion process that produces objects of mass.

    In this case, general relativity’s curvature of spacetime expressed in abstract dimensional coordinates used to describe the effects of gravitation accurately represents gradients of vacuum energy density.

    This proposal requires that vacuum energy density correlates precisely with the magnitude of local gravitational effects. As a test, I suggest that quantum fluctuations (the annihilation of virtual particle/antiparticle pairs) are an indirect measure of vacuum energy density, and that their rate of occurrence should vary (to some perhaps very small extent) with radial distance from the Earth’s center of mass, including at orbital distances.

    I think this proposal provides a physical mechanism for producing the gravitational effects described as dimensional coordinates of spatial geometry in GR. It also explains why gravitation, as a property of spacetime, seems so much ‘weaker’ than forces of matter, despite the unlimited range of its effects. In the absence of matter to locally redirect and contract vacuum energy, the kinetic vacuum energy disperses in accordance with the second law of thermodynamics, universally expanding spacetime. The physical nature of spacetime is the result of its initial infusion of universal vacuum energy – that which had not been condensed to form particles of persistent matter.

    Link to this
  6. 6. basudeba 12:45 am 11/15/2012

    Dear Sir,
    When you say; “It seems to me that if matter (gravitation) had dominated universal development there would have been no expansion”, you equate matter with gravitation. But it may not be case always. The force varies with distance at a particular rate means that if the matter (mass) is static, the force would not be felt. If the force is felt even weakly, as is the case for gravitation, the mass is mobile, which means, it is mv, i.e., momentum. Thus, gravitation is not related directly to mass, but through momentum. This velocity component is often ignored. Since this velocity requires some force, that force explains the initial expansion of the universe”. The question is what is the source of this initial force? We have a theory for that which we will discuss separately.

    How gravitation slows down the expansion can be explained if we treat the so-called smooth and persistent “dark energy” as the background structure. Since it is “dark” – does not interact with matter – it cannot be energy, as energy is not directly perceived, but is perceived only through its interaction with matter. Thus, any motion through it will cause friction and produce a bow-shock effect to slow it down with distance.

    When you say: “The universal effect of gravitation is to locally condense matter”, you are right, but it is not the whole effect. This condensation is not infinite, but only up to a point, where the components or bodies stabilize. The components of the body stabilize in a specific manner to create the body as it is. The bodies stabilize in a specific manner to order the universe as it is. This stabilization depends upon the individual density vis-à-vis total field density. It is like dumping a huge quantity of salt on the shore of a river. The salt melts and increases the local density of water. A boat passing by will be deflected by this increase in local density. This deflection will be proportional to the relative density and will fall off with distance. Since while moving, the boat sequentially crosses the planes at its front, and since the plane is a two dimensional structure, the variation will show a second order effect.

    If you imagine the galaxies as revolving on a universal axis at different velocities at different distances like the planets are moving in the solar system, their speeding away can be explained as a temporary phenomenon. No need for dark energy.
    Regards,
    basudeba

    Link to this
  7. 7. yashten 9:28 pm 11/15/2012

    Dear fellows,

    Please forgive me for not being in harmony with this;

    “Today’s universe is expanding. Not just that, its expansion is accelerating. Galaxies are not only getting further away from us all of the time, but they’re doing so faster and faster with each passing day.”

    It is only a matter, I have a concept probably you won’t like that as well.

    This is mine.
    But to me, that energy wasn’t & isn’t dark. We can watch its effect. We can trace its source. Its source (N) hides around behind Ursa Major & Leo constellation. It has pushed low redshift galaxies/quasars (z around 5) back toward it. The appearing of other at least 7 high redshift quasars (z > 6) at north of Galactic Coordinate System also support that. It leaves some of high redshift galaxies/quasars (z > 5) at south of Galactic Coordinate System, as signal of belated light phenomenon. And the appearing of NGC 3314a & NGC 3314b at Hydra, http://en.wikipedia.org/wiki/List_of_galaxies, also gives another positive signal of belated light phenomenon. It is expected on surface of ball with diameter MW-N. And plus ghost galaxy at Leo Minor, http://astrobob.areavoices.com/tag/ic-2497/.

    Too many festivity there, around Ursa Major & Leo, so a lot of high redshift sky objects were & are intrigued to get there.

    Link to this
  8. 8. yashten 9:39 pm 11/15/2012

    Its source (N) hides around behind Ursa Major & Leo constellation. It has pushed low redshift galaxies/quasars (z around 5) back toward it. The appearing of other at least 7 high redshift quasars (z > 6) at north of Galactic Coordinate System also support that. It leaves some of high redshift galaxies/quasars (z > 5) at south of Galactic Coordinate System, as signal of belated light phenomenon.

    Link to this
  9. 9. yashten 9:40 pm 11/15/2012

    It has pulled at least 10 high redshift quasars (z > 5) back toward it.

    Link to this
  10. 10. yashten 10:59 pm 11/15/2012

    Something communication system has changed messages I sent before. Different I wrote, different appeared.

    It should be written like this:
    It has pushed low redshift galaxies/quasars (z around 5) back toward it. The appearing of other at least 7 high redshift quasars (z > 6) at north of Galactic Coordinate System also support that.

    Link to this
  11. 11. yashten 11:00 pm 11/15/2012

    No! It isn’t what I wrote!

    Link to this
  12. 12. yashten 11:06 pm 11/15/2012

    It has pushed low redshift galaxies/quasars (Z around 5) back toward it. The appearing of…..

    Link to this
  13. 13. yashten 11:11 pm 11/15/2012

    Why? somebody mess up my comment? afraid of current theory being dead?

    Link to this
  14. 14. yashten 11:35 pm 11/15/2012

    On PHYS.ORG as it is as I wrote.

    Link to this
  15. 15. Kelly Oakes in reply to Kelly Oakes 2:59 am 11/16/2012

    yashten: your comment was stuck in moderation because of the links you included. I’ve let it through this time, but I’d like to ask everyone to try to stay on topic (the specific topic of this post) and not write to too great a length. In future I’ll moderate comments that don’t stick to this.

    Link to this
  16. 16. basudeba 6:02 am 11/16/2012

    Dear Sir,
    Some people simply cannot think of beyond text books. They try to discourage new ideas and sometimes try to delete or mutilate it. So have a heart. Truth will ultimately prevail.
    Regards,
    basudeba

    Link to this
  17. 17. vinodkumarsehgal 11:20 am 11/16/2012

    I agree with Keilly Oakes that under a public forum one should stay limited to topic of article and comments should also be brief. But there are some concepts over which scientific community itself is divided. Even prominent scientists lack complete clarity on such controversial subjects. Dark energy, expansion of universe, gravitation are some of such controversial subjects. When articles on such subjects, as written by scientific reporters or scientists but not engaged directly in research on such subjects, appear in popular scientific magazines, readers can’t resist themselves from expressing their frank and candid views on main subject or related issues. For the sake of development of scientific spirit, candid and unrestricted discussion should be encouraged rather than restricted. Leave aside accelerated expansion, my comments on physicality of expansion were under that context.

    Link to this
  18. 18. ChrisG5000 6:20 pm 11/16/2012

    Could gravity alone be used to explain the expansion of the universe?

    What if “Our Universe” is a large area of space which contains an expanding set of matter, and what if the “Real Total Universe” is made up of an infinite collection of such areas, too remote for us to see, or which may no longer even contain emitting stars. Some are expanding and others collapsing. Given this hypothetical scenario, has anyone ever done the maths on what happens as “Our Universe” starts to feel gravitational pull from its neighbouring universes, since its expansion must bring it closer to them? Surely we would expect to see an accelerating expansion?

    Link to this
  19. 19. Kelly Oakes in reply to Kelly Oakes 2:59 pm 11/17/2012

    Hi ChrisG5000: it’s certainly an intriguing idea. there’s something similar suggested in this article from 2010 that you might find interesting: http://news.nationalgeographic.co.uk/news/2010/03/100322-dark-flow-matter-outside-universe-multiverse/

    Link to this

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