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Gravitational attraction between galaxies is slowing the expansion of the universe, but the amount of slowing down is not known directly. In fact, the cosmological constant can counteract gravity’s effect. As recent measurements indicate, the universe is expanding faster now than in the past—perhaps a “modern inflationary era” in which the dark energy is thought to be causing the expansion of the present-day universe to accelerate. If the expansion rate were affected by gravity alone, we should be able to see that the expansion rate between distant galaxies was once greater than it is now. However, measurements show it was less than now. We can, however, calculate the amount of slowing based on the average density of matter we observe directly. Here we have a definite answer—there is far less visible matter than needed to stop expansion. The critical density     ρ c size 12{ρ rSub { size 8{c} } } {} is defined to be the density needed to just halt universal expansion in a universe with no cosmological constant. It is estimated to be about

ρ c 10 26 kg/m 3 . size 12{ρ rSub { size 8{c} } approx "10" rSup { size 8{ - "26"} } `"kg/m" rSup { size 8{3} } "."} {}

However, this estimate of ρ c size 12{ρ rSub { size 8{c} } } {} is only good to about a factor of two, due to uncertainties in the expansion rate of the universe. The critical density is equivalent to an average of only a few nucleons per cubic meter, remarkably small and indicative of how truly empty intergalactic space is. Luminous matter seems to account for roughly 0 . 5% size 12{0 "." 5%} {} to 2% size 12{2%} {} of the critical density, far less than that needed for closure. Taking into account the amount of dark matter we detect indirectly and all other types of indirectly observed normal matter, there is only 10% size 12{"10"%} {} to 40% size 12{"40"%} {} of what is needed for closure. If we are able to refine the measurements of expansion rates now and in the past, we will have our answer regarding the curvature of space and we will determine a value for the cosmological constant to justify this observation. Finally, the most recent measurements of the CMBR have implications for the cosmological constant, so it is not simply a device concocted for a single purpose.

After the recent experimental discovery of the cosmological constant, most researchers feel that the universe should be just barely open. Since matter can be thought to curve the space around it, we call an open universe negatively curved    . This means that you can in principle travel an unlimited distance in any direction. A universe that is closed is called positively curved    . This means that if you travel far enough in any direction, you will return to your starting point, analogous to circumnavigating the Earth. In between these two is a flat (zero curvature) universe    . The recent discovery of the cosmological constant has shown the universe is very close to flat, and will expand forever. Why do theorists feel the universe is flat? Flatness is a part of the inflationary scenario that helps explain the flatness of the microwave background. In fact, since general relativity implies that matter creates the space in which it exists, there is a special symmetry to a flat universe.

Figure a shows an eye looking toward a rotating spiral galaxy. Light coming from the galaxy arms that are rotating away from the observer is red shifted, and light coming from the galaxy arms that are rotating toward the observer is blue shifted. Figure b shows a graph of star rotation speed versus star radius. From zero radius, the curve increases steeply and peaks near two hundred and fifty kilometers per second at a radius of zero point five arbitrary units. The curve then decreases slightly and progresses to a radius of sixteen arbitrary units with ups and down between two hundred and two hundred and twenty five kilometers per second. Figure c shows an image of multiple galaxies and other objects against a black background. In the center of the image is a roughly circular purple haze, on the left and right of which are two slightly smaller and roughly circular blue haze regions.
Evidence for dark matter: (a) We can measure the velocities of stars relative to their galaxies by observing the Doppler shift in emitted light, usually using the hydrogen spectrum. These measurements indicate the rotation of a spiral galaxy. (b) A graph of velocity versus distance from the galactic center shows that the velocity does not decrease as it would if the matter were concentrated in luminous stars. The flatness of the curve implies a massive galactic halo of dark matter extending beyond the visible stars. (c) This is a computer-generated image of X rays from a galactic cluster. The X rays indicate the presence of otherwise unseen hot clouds of ionized gas in the regions of space previously considered more empty. (credit: NASA, ESA, CXC, M. Bradac (University of California, Santa Barbara), and S. Allen (Stanford University))

Questions & Answers

how i don understand
Willam Reply
how do I access the Multiple Choice Questions? the button never works and the essay one doesn't either
Savannah Reply
How do you determine the magnitude of force
Peace Reply
mass × acceleration OR Work done ÷ distance
Seema
Which eye defect is corrected by a lens having different curvatures in two perpendicular directions?
Valentina Reply
acute astigmatism?
the difference between virtual work and virtual displacement
Noman Reply
How do you calculate uncertainties
Ancilla Reply
What is Elasticity
Salim Reply
using a micro-screw gauge,the thickness of a piece of a A4 white paper is measured to be 0.5+or-0.05 mm. If the length of the A4 paper is 26+or-0.2 cm, determine the volume of the A4 paper in: a). Cubic centimeters b). Cubic meters
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what is module
Alex Reply
why it is possible for an object(man) to stay on air without falling down?
akande Reply
its impossible, what do you mean exactly?
Ryan
Exactly
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it's impossible
Your
Why is it not possible to stand in air?
bikko
the air molecules are very light enough to oppose the gravitational pull of the earth on the man..... hence, freefall occurs
Arzail
what is physics
Joshua Reply
no life without physics ....that should tell you something
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E=MC^2
study of matter and energy and an inter-relation between them.
Minahil
that's how the mass and energy are related in stationery frame
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Kinetic energy is the energy due to montion of waves,electrons,atoms, molecule,substances an object s.
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plz anyone can tell what is meteor and why meteor fall in night? can meteor fall in the day
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meteor are the glowy (i.e. heated when the enter into our atmosphere) parts of meteoroids. now, meteoroids are the debris resulting from the collision of asteroids or comets. yes, it occurs in daytime too, but due to the daylight, we cant observe it as clearly as in night
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Waka
wich method we use to find the potential on a grounded sphere
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with out a physics the life is nothing to see
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What do you want to talk about😋😋
Emmanuella
the study of all the natural events occuring around us..... this is Physics (until those events obey the laws of physics)
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Conservation of energy😰
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yeah, that too
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Energy, it always remains there in a physical system. it can only take the form either in motion (kinetic energy) or in rest (potential energy)
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In nature organisms feed on one another in an orderly way.
Emmanuella
that describes the food chain, in which we humans are at the top
Arzail
The energy that came initially from the sun 🌞is converted into a form in which it can be stored in green plant.
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Therefore, there is conservation of energy.
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x-rays are electromagnetic Ray's produced when electrons with very high acceleration is brought to a stop by a target metal..
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determine how much less the mass of lithium with mass number of 7 and proton of 3 nucleus is compared to that of its constituents.the mass of neutral Li 6.015123 u, calculate the total binding energy and the binding energy per nucleon
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physics is a natural sciences that deals with the study of matter (solid, liquid gases) and energy (heat, thermal)
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physics is the study of natural phenomena it deals with matter and energy and the relationship that exists between them
Eunice
yeah
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physics is the study of quantities and there measurements
Christine
physics is the study of natural phenomena that deals with all the three states of matter
Wilson
I'm with Eunice Femi
Prince
@ wilson, matter exist in more than three states.
adewunmi
more than three states you say @Adewunmi which are?
Christine
@VICTOR the study of natural events around us, known as Physics
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@Christine plasma, which is hot and ionised gas and has become conductive
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Source:  OpenStax, College physics. OpenStax CNX. Jul 27, 2015 Download for free at http://legacy.cnx.org/content/col11406/1.9
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