In an expanding Universe, Space and Time are stretched. Is Dark Energy responsible for this?

The discovery in 1998 that the Universe is actually speeding up its expansion was a total shock to astronomers. It just seems so counter-intuitive, so against common sense. But the evidence has become convincing.

The evidence came from studying distant Type Ia supernovae. This type of supernova results from having a white dwarf star in a binary system. Matter transfers from the normal star to the white dwarf until the white dwarf attains a critical mass (the Chandrasekhar limit) and undergoes a thermonuclear explosion. Because all white dwarfs achieve the same mass before exploding, they all achieve the same luminosity and can be used by astronomers as "standard candles." Thus by observing their apparent brightness, astronomers can determine their distance using the 1/r2 law.

By knowing the distance to these supernovae, we know how long ago they occurred. In addition, the light from the supernova has been red-shifted by the expansion of the Universe. By measuring this redshift from the spectrum of the supernova, astronomers can determine how much the Universe has expanded since the explosion. By studying many supernovae at different distances, astronomers can piece together a history of the expansion of the Universe.

In the 1990's two teams of astronomers, the Supernova Cosmology Project (Lawrence Berkeley National Laboratory) and the High-Z Supernova Search (international), were looking for distant Type Ia supernovae in order to measure the expansion rate of the Universe with time. They expected that the expansion would be slowing, which would be indicated by the supernovae being brighter than their redshifts would indicate. Instead, they found the supernovae to be fainter than expected. Hence, the expansion of the Universe was accelerating!

In addition, measurements of the cosmic microwave background indicate that the Universe has a flat geometry on large scales. Because there is not enough matter in the Universe -- either ordinary or dark matter -- to produce this flatness, the difference must be attributed to a "dark energy". This same dark energy causes the acceleration of the expansion of the Universe. In addition, the effect of dark energy seems to vary, with the expansion of the Universe slowing down and speeding up over different times.

Astronomers know dark matter is there by its gravitational effect on the matter that we see, and there are ideas about the kinds of particles it must be made of. By contrast, dark energy remains a complete mystery. The name "dark energy" refers to the fact that some kind of "stuff" must fill the vast reaches of mostly empty space in the Universe in order to be able to make space accelerate in its expansion. In this sense, it is a "field" just like an electric field or a magnetic field, both of which are produced by electromagnetic energy. But this analogy can only be taken so far, because we can readily observe electromagnetic energy via the particle that carries it, the photon.

Some astronomers identify dark energy with Einstein's Cosmological Constant. Einstein introduced this constant into his general relativity when he saw that his theory was predicting an expanding universe, which was contrary to the evidence for a static universe that he and other physicists had in the early 20th century. This constant balanced the expansion and made the Universe static. With Edwin Hubble's discovery of the expansion of the Universe, Einstein dismissed his constant. It later became identified with what quantum theory calls the energy of the vacuum.

In the context of dark energy, the cosmological constant is a reservoir which stores energy. Its energy scales as the Universe expands. Applied to the supernova data, it would distinguish effects due to the matter in the Universe from those due to the dark energy. Unfortunately, the amount of this stored energy required is far more than observed and would result in very rapid acceleration -- so much so that the stars and galaxies would not form. Physicists have suggested a new type of matter, "quintessence," which would fill the Universe like a fluid which has a negative gravitational mass. However, new constraints imposed on cosmological parameters by Hubble Space Telescope data rule out at least simple models of quintessence.

Other possibilities being explored are topological defects, time-varying forms of dark energy, or a dark energy that does not scale uniformly with the expansion of the Universe.

Well, dark energy is only a hypothetical concept. We observe that galaxies hold together more than what their mass should do and we call it, dark matter. We observe that the universe expands at a rate that is too great for its energy and we call it, dark energy.

You must see it as a "place holder" until we find a better definition. It makes sense in an equation on paper but it might be hiding something entirely different.

Does space and time stretch? If so, stretch in relation to what? The problem with the universe is that, we are part of it and we can't observe it from "outside." We know that space and time are related but how? Scientists like Georg Cantor and Kurt Goedel nearly lost their mind, trying to find the true nature of time.

Say, I had a measuring tape extending from here to a distant galaxy that recess from us, according to the expansion and its red-shift, what happens?

a) Is the distant galaxy moving away from the end of my tape, or

b) is space creates along the tape?

If a) then it is not an expansion but an explosion and distant galaxies are moving into something! If b) then where are say, the Plank's units of space created? At the end of my tape? The beginning? Every mile of it?

As you see, things are very simple on earth. René Descartes wrote that the straight line is the shortest distance between two points. But what is a "straight" line at the scale of the universe?

I am afraid that we have still more questions than answers, my friend!

Time cannot be stretched as it is neither material nor energy. It can be slowed down or sped up within a relatively small field of perspective under specific circumstances. Scientists theorize that dark energy is the reason why space may be expanding. Space can expand or stretch because it is made of energy. The fabric of space is made of threads of energy in a network similar to a piece of fabric with warp and woof threads that can be stretched or spread out. Time is the measure of the motion of energy and thus is an abstract concept or perception sensed in the changes that result from the constant motion of energy as we perceive them. Energy is not so much consumed but is constantly transforming from one form to another.

Scientists do theorize that what is called dark energy is responsible for the expansion of the universe. We will have to wait until future examination and evidence.

This seems to to be a commonly held misconception.

But space and time are not stretched. New space is not being created inside the universe.

The universe is expanding.

At the edge of the universe this means that objects are moving into new areas of space. Science is not able to explain the conditions at the edge of the universe. Indeed there is doubt about the universe having an edge.

Within the universe objects are moving with a relative velocity that is carrying most objects away from us.

The speed that objects are receding from us is roughly proportional to their distance from us.

Yes it expanding with time.But the expansion is not due to dark matter it is due to Big Bang.As long as KE of the universe is greater than potential energy it will keep on expanding(KE was supplied by Big Bang event)

IVAN

short answer yes. nothing new to add to previous answers.