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The Universe has always been expanding since the big bang. Today the expansion is accelerating. Has there been any point in time when the expansion of the universe was decelerating? How do we know that? We also know that accelerated space expansion requires the "pumping" by dark energy. Does decelerated expansion require any such cause? Thanks!

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    $\begingroup$ The universe expanded extremely rapidly shortly after big bang. The fact that it expanded faster then than it does now implies that at some point expansion did decelerate. $\endgroup$ – Simon Jun 1 '18 at 19:52
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    $\begingroup$ Whether the universe is accelerating depends on the cosmological model, which is a theory that should explain observations. The current official model does not explain observations, unless we pretend that things exist that we don't observe (e.g. "dark energy", "inflation", "infinite mass", etc.). There are more holes in this model than in a slice of a Swiss cheese. In other models the universe does not accelerate and there is no need for the imaginary "dark energy" to explain the expansion. $\endgroup$ – safesphere Jun 1 '18 at 21:32
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Was the expansion of the universe ever decelerating?

The Universe has always been expanding since the big bang. Today the expansion is accelerating. Has there been any point in time when the expansion of the universe was decelerating? How do we know that? We also know that accelerated space expansion requires the "pumping" by dark energy. Does decelerated expansion require any such cause?

The Berkeley webpage "BOSS Quasars Unveil a New Era in the Expansion History of the Universe" has this image and caption:

BOSS Quasar measurements show early expansion rate

"Until recently, three-dimensional maps by BOSS and other surveys were able to measure the regular distribution of galaxies back to an average of only about five and a half billion years ago, a time when the expansion of the universe was already accelerating. BOSS’s quasar measurements (red circle, left), by measuring the distribution of intergalactic gas, have now probed the structure of the early universe at a time when expansion was still slowing under the influence of gravity. The quasar data gives new access to the transition from deceleration to acceleration caused by dark energy. (Graph by Zosia Rostomian, Lawrence Berkeley National Laboratory, and Nic Ross, BOSS Lyman-alpha team, Berkeley Lab)"

See also: "The clustering of galaxies in the SDSS-III Baryon Oscillation Spectroscopic Survey: Baryon Acoustic Oscillations in the Data Release 9 Spectroscopic Galaxy Sample" (29 Mar 2012), by Anderson, Aubourg, Bailey, Et Al.:

"1 INTRODUCTION

Explaining the late-time acceleration of the expansion rate of the Universe (Riess et ali. 1998; Perlmutter et al. 1999) is one of the most perplexing problems in modern physics. All known attempts require exotic ingredients: a new, very small energy scale in a cosmological constant or low-mass field, a change to General Relativity to weaken gravity on large scales or at low densities, or extra dimensions of space-time. Empirical observations will provide clues as to the cause by providing precision measurements of the expansion history and the growth of cosmological structure over time (e.g. Kolb et al. 2006).

One of the key methods for measuring the expansion history is to use features in the clustering of galaxies within galaxy surveys as a ruler with which to measure the distance–redshift relation. Obtaining precision distance measurements is a long-standing challenge in astronomy, and the Baryon Acoustic Oscillations (BAO) signal in the two-point clustering of galaxies provides a particularly robust quantity to measure. The BAO arise because the coupling of baryons and photons by Thomson scattering in the early universe allows acoustic oscillations at early times, which in turn leads to a rich structure in the distribution of matter and the anisotropies of the cosmic microwave background (CMB) radiation. The distance that acoustic waves can propagate in the first million years of the universe becomes a characteristic comoving scale ...".

SDSS III's webpage "BOSS: Dark Energy and the Geometry of Space" explains:

"... When the survey is completed, the errors on the BOSS measurements will be further reduced, and we will also measure the BAO at several other redshifts.

Using the acoustic scale as a physically calibrated ruler, BOSS will determine the angular diameter distance with a precision of 1% at redshifts z = 0.3 and z = 0.55 using the distribution of galaxies. It will also measure the distribution of quasar absorption lines at z = 2.5, yielding a measurement of the angular diameter distance at that redshift to an accuracy of 1.5%. It will also measure the cosmic expansion rate H(z) with 1-2% precision at the same redshifts. These measurements will provide demanding tests for theories of dark energy and the origin of cosmic acceleration.".

Press Release: "Astronomers from the Sloan Digital Sky Survey Make the Most Precise Measurement Yet of the Expanding Universe" (April 7, 2014):

"... Delubac explains that "we have measured the expansion rate in the young Universe with an unprecedented precision of 2 percent." Measuring the expansion rate of the Universe over its entire history is key in determining the nature of the dark energy that is responsible for causing this expansion rate to increase during the past six billion years. "By probing the Universe when it was only a quarter of its present age, BOSS has placed a key anchor to compare to more recent expansion measurements as dark energy has taken hold," says Delubac.

BOSS determines the expansion rate at a given time in the Universe by measuring the size of baryon acoustic oscillations (BAO), a signature imprinted in the way matter is distributed, resulting from sound waves in the early Universe. This imprint is visible in the distribution of galaxies, quasars, and intergalactic hydrogen throughout the cosmos.

...

With enough good quasar spectra, close enough together, the position of the gas clouds can be mapped in three dimensions. BOSS determines the expansion rate by using these maps to measure the size of the BAO pattern at different epochs of cosmic time. These new measurements provide key data for astronomers seeking the nature of the dark energy postulated to be driving the increase in the expansion rate of the Universe.".

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