# Cosmology Forum Abbreviations

1. Sep 21, 2012

I’m a layman new to the Cosmology Forum and just finished reading through the very helpful, “Effort to get us all on the same page (balloon analogy anyone?)” by Marcus et. al. followed by “Look 88 billion years…”. [If you’re in a hurry see the nice summary by Mark M at cosmo.pdf.]

Along the way, I kept a list of some commonly encountered abbreviations which I post below. My intention is to develop an abbreviation list as follows:

1. Anyone interested in adding appropriate abbreviations would do so by listing them in a post. Feel free to copy and paste abbreviations you encounter while perusing these threads (preferably with expansions). A nice addition for example, might be the acronyms for various satellites busy (or planned) obtaining relevant data.

2. Similarly, post corrections to those I have already listed. This is just a starting point. I’m sure I’ve botched many of these, so don’t hesitate to set things straight.

3. I will periodically collect the suggestions, amend the list and repost it so the most current list will appear near the last page of this thread.

Hopefully, a Cosmology Forum authority will occasionally copy the current list, edit ad lib and make a sticky post or FAQ out of it that anyone can easily refer to.

As the Physics Forums generally discourages abbreviations, the aim is a brief useful list. This list is not meant to be a dictionary of precise definitions. It’s meant to be a quick reference, decoding abbreviations with a line or two (at most) of explanation. If a dictionary of definitions is desired, that should be a separate thread. Don’t be shy in volunteering abbreviations and please, don’t feel insulted if your suggestion is not adopted. You’re effort is appreciated either way.

P.S. If this list is either unwanted, disallowed, stupid, or already exists, a Cosmology authority need only post “no thanks” and I will abandon the effort. (It’s OK with me if the entire thread is deleted.)

Last edited: Sep 21, 2012
2. Sep 21, 2012

$\Lambda$CDM lambda cold dark matter, a.k.a. LCDM

$\Lambda$ greek “lambda”, symbolizes spacetime curvature in Einstein’s field equations (EFE), $\Lambda =$8$\pi$G$\rho$de

Δ Greek “delta”, indicating “change in” or “displacement”

Δt temporal displacement as indicated on the t-coordinate axis

Δx spatial displacement as indicated on the x-coordinate axis

Δy spatial displacement as indicated on the y-coordinate axis

Δz spatial displacement as indicated on the z-coordinate axis

* multiply operator, e.g. 2*3 = 6

^ exponent operator, e.g. 2^3 = 8

3D three dimensional or third dimension, hyphen is optional, ex. 3-D.

$\rho$ Greek “rho”, the mass-equivalent density of total matter and energy (of all types) per unit volume. Typically excludes the cosmological constant (dark energy). Often excludes contribution from radiation as insignificant from a relatively short time after the Big Bang event. Value now ≈ 0.228 nanojoule per m3, about 16% ordinary matter, the rest is mostly dark matter

$\rho$m density attributable to matter

$\rho$de density attributable to dark energy

a the function a(t), when used in a mathematical expression.

a’ 1st derivative with respect to time of scale factor a(t)

a’’ 2nd derivative with respect to time of scale factor a(t)

a(t) scale factor , default a(now) = 1, by convention

BA balloon analogy as it relates to expansion

BB Big Bang event

BLY billion light years (preferred to the equivalent GLY, giga-light years).

c universal speed limit exhibited by uninhibited “massless” particle including light and presumably gravity, $c\ =\ 2.99792458\ \times\ 10^{8}\ m\ s^{-1}$

d 108 years, a convenient cosmology time unit, e.g. H(now) = 1/139 per d.

DE dark energy, a.k.a. cosmological constant

DM dark matter

EFE Einstein field equations

frame frame of reference, inertial (non-accelerating) unless otherwise specified.

G gravitational constant

GLY giga-light years, BLY (billion light years) is preferred.

GR general relativity, Einstein’s 1915 theory of.

H Hubble expansion rate. a.k.a. “Hubble parameter”, “Hubble constant” (now known to vary slightly over time). The distance between observers at rest grow at this rate-a certain fraction or percent of their length per unit time. The fractional increase in scale factor (a), H = a’/a. Default value H(now) = 1/139 of one percent per million years.

h Planck’s constant, $h\ =\ 6.62606876(52)\ \times\ 10^{-34}\ J\ s$.

ħ reduced Planck’s constant, ħ = h/2$\pi$

H$\infty$ Hubble parameter’s final asymptotic value, H$\infty$ = 1/163 per d = 1/163 of one percent per million years

CDM cold dark matter

LY light year, distance traveled by unhindered light in one earth year

M2 Minkowski diagram, a subset of M4 restricted to 1 spatial (horizontal) and 1 temporal (vertical) dimension.

M4 Minkowski spacetime, refers to 3 spatial plus 1 temporal orthogonal dimensions exhibiting relationship consistent with Einstein’s general theory of relativity (GR).

MLY million light years

Mpc mega parsec, a spatial distance, 3.26 million light years.

nD nth dimension, where n indicates the number of dimensions referenced

PF Physics Forums

Q.E.D. quod erat demonstrandum, mathematical expression meaning “thus it is demonstrated” (periods required)

QED quantum electrodynamics

QFT quantum field theory

QM quantum mechanics, quantum theory, quantum physics

R current estimated radius of the universe as modeled in the balloon analogy, xxx billion light years

sqrt the square root operator (√), e.g. sqrt(9) = 3

SR special relativity, Einstein’s 1905 theory of.

t time, default = now

U universe, most often all of three dimensional space and its contents at a particular time, default = now.

WMAP Wilkinson Microwave Anisotropy Probe

z fractional amount distances and wavelengths have increased while light was in transit. Arriving wavelength is 1+z times original.

Last edited: Sep 21, 2012
3. Sep 21, 2012

### Staff: Mentor

1./2. derivative of a (with respect to time for the scale factor)

108? Can be written as [noparse]108[/noparse]

Your pi and lambda below planck's constant look broken.

I would expect ly for light year.
Is there any reason to prefer Bly, where Gly follows the SI prefixes?

Observable universe? ~46 Gly I think.

[noparse]Wilkinson Microwave Anisotropy Probe[/noparse]

Usually, the long version of the names is not really relevant anyway. For example, Gaia is "Global Astrometric Interferometer for Astrophysics", but the final design does not use an interferometer.

4. Sep 21, 2012