Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: On the least action principle in cosmology   Given the present distribution of mass tracing objects in an expanding
universe, we develop and test a fast method for recovering their past orbits
using the least action principle. In this method, termed FAM for Fast Action
Minimization, the orbits are expanded in a set of orthogonal time-base
functions satisfying the appropriate boundary conditions at the initial and
final times. The conjugate gradient method is applied to locate the extremum of
the action in the space of the expansion coefficients of the orbits. The
TREECODE gravity solver routine is used for computing the gravitational fields
appearing in the action and its gradients. The time integration of the
Lagrangian is done using Gaussian quadratures. FAM allows us to increase the
number of galaxies used in previous numerical action principle implementations
by more than one order of magnitude. For example, orbits for the $\sim 15,000$
IRAS PSC$z$ galaxies can be recovered in $\sim 12,000$ CPU seconds on a 400MHz
DEC-Alpha machine. FAM can recover the present peculiar velocities of particles
and the initial fluctuations field. It successfully recovers the flow field
down to clusters scales, where deviations of the flow from the Zel'dovich
solution are significant. We also show how to recover orbits from the present
distribution of objects as in redshift space by direct minimization of a
modified action, without iterating the solution between real and redshift
spaces.

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The categories of this paper are: atom-ph
3
The categories of this paper are: physics
29
The categories of this paper are: funct-an
11
The categories of this paper are: hep-lat
24
The categories of this paper are: dg-ga
35
The categories of this paper are: mtrl-th
33
The categories of this paper are: plasm-ph
10
The categories of this paper are: q-bio
27
The categories of this paper are: comp-gas
31
The categories of this paper are: ao-sci
20
The categories of this paper are: acc-phys
15
The categories of this paper are: nucl-ex
12
The categories of this paper are: nucl-th
26
The categories of this paper are: bayes-an
16
The categories of this paper are: stat
21
The categories of this paper are: adap-org