A&A 450, 899-907 (2006)
DOI: 10.1051/0004-6361:20053706
N. J. Nunes1 - A. C. da Silva2,3 - N. Aghanim2
1 - School of Physics and Astronomy, 116 Church Street S.E.,
University of Minnesota,
Minneapolis,
Minnesota 55455,
USA
2 - IAS - CNRS, Bâtiment 121, Université Paris-Sud, 91405 Orsay,
France
3 -
Centro de Astrofíisica da Universidade do Porto, Rua das Estrelas,
4150-764 Porto, Portugal
Received 27 June 2005 / Accepted 6 October 2005
Abstract
In the simple case of a constant equation of state, redshift
distribution of collapsed structures may constrain dark energy models.
Different dark energy models having the
same energy density today but different equations of state give quite
different number counts. Moreover, we show that introducing the
possibility that dark energy collapses with dark matter
("inhomogeneous'' dark energy) significantly complicates the picture. We
illustrate our results by comparing four dark energy models to the
standard -model. We investigate a model with a constant
equation of state equal to -0.8, a phantom energy model and two scalar
potentials (built out of a combination of two exponential
terms). Although their equations of state at present are almost
indistinguishable from a
-model, both scalar potentials
undergo quite different evolutions at higher redshifts and give
different number counts. We show that phantom dark energy induces
opposite departures from the
-model as compared with the
other models considered here. Finally, we find that inhomogeneous dark
energy enhances departures from the
-model with maximum
deviations of about 15% for both number counts and integrated number
counts. Larger departures from the
-model are obtained for
massive structures which are rare objects making it difficult to
statistically distinguish between models.
Key words: cosmology: cosmological parameters - galaxies: clusters: general
In the present general picture of cosmology, converging evidences
suggest that the matter density parameter is low and that the largest
fraction of the energy density of the universe has an unknown nature
leading to an accelerating phase. These indications come primarily
from supernovae Ia data
(e.g. Perlmutter et al. 1999; Riess et al. 2004,1998) and are
corroborated by cosmic microwave background radiation
(e.g. Spergel et al. 2003) and large scale structure observations
(e.g. Cole et al. 2005), although there are different interpretations
for the data (e.g. Shanks 2004; Blanchard et al. 2003). A
cosmological constant can explain the acceleration of the universe,
however, the disagreement by 120 orders of magnitudes with
predictions from theoretical particle physics has shown the need for
resorting to an alternative explanation. This is why several
theoretical models were recently proposed to explain this dark energy
in the universe. This new component can be identified with a slowly
varying, self-interacting, neutral scalar "quintessence'' field
(Ferreira & Joyce 1997; Ratra & Peebles 1988; Wetterich 1988; Zlatev et al. 1999)
which can be minimally coupled, non-minimally coupled
(e.g. Amendola 2000; Baccigalupi et al. 2000; Uzan 1999), a
phantom (e.g. Caldwell 1999), a tachyon
(e.g. Bagla et al. 2003) or of purely kinetic nature
(e.g. Armendariz-Picon et al. 2000), known as K-essence. An
alternative to dark energy is a fluid with a Chaplygin gas type of
equation of state (e.g. Kamenshchik et al. 2001). See
Sahni (2004) and references therein for a review on some of
these, and other, models.
Regardless of its nature, dark energy as a dominant component, plays a role in the structure formation and thus is likely to modify the number of formed structures. The evolution of linear perturbations in a scalar field like quintessence and the effects on structure formation have already been investigated theoretically (e.g. Ferreira & Joyce 1997; Amendola 2003; Perrotta & Baccigalupi 2002). The effects on the abundance of collapsed structures and its evolution with redshift were also widely explored and suggested as a tool to constrain the dark energy's nature and evolution (e.g. Weinberg & Kamionkowski 2002; Weller et al. 2002; Mohr 2004; Horellou & Berge 2005; Haiman et al. 2001; Battye & Weller 2003; Wang et al. 2004).
Recently, numerical simulations including a dark energy component were performed by several groups to complement the analytical computations and to study the effects of dark energy at the structure level (e.g. shape of the dark matter halo, mass function) (Linder & Jenkins 2003; Kuhlen et al. 2004; Lokas et al. 2003; Klypin et al. 2003). In such studies, the scalar field associated with dark energy is assumed not to have density fluctuations on scales of galaxy clusters or below. If dark energy influences the perturbations on small scales as proposed for example by Arbey et al. (2001), Bean & Magueijo (2002), Padmanabhan & Choudhury (2002) or Bagla et al. (2003), the collapse of structures as well as their properties will be affected.
Mota & van de Bruck (2004) have shown that the properties of collapsed halos
(density contrast, virial radius) depend strongly on the shape of the
potential, the initial conditions, the time evolution of the equation
of state and on the behaviour of the scalar field in non-linear
regions. This is what we will refer to as the inhomogeneity of
the scalar field. More recently, Nunes & Mota (2004) have investigated
how inhomogeneous quintessence models have a specific signature even
in the linear regime of structure formation. They have shown that the
time of collapse is affected by the inhomogeneity of dark energy and
they have computed the resulting effect on the linearly extrapolated
density threshold
.
Moreover, they examined the evolution of
matter overdensity as a function of time varying equation of state in
homogeneous and inhomogeneous assumptions. Maor & Lahav (2005) have
generalized the formalism to allow for a smooth transition between the
homogeneous and inhomogeneous scenarios. They have concluded
that, if only matter virializes, the final size of the system is
fundamentally distinct from the one reached if the full system
virializes.
In the present study, we extend the work of Nunes & Mota (2004) to
investigate how the quintessence field affects the abundance of
collapsed halos when the field follows the background evolution
(homogeneous) and more specifically when it collapses with the dark
matter (inhomogeneous). We compare the two assumptions for models with
constant equation of state and more general cases of time-varying
equation of state. To compute the structure abundances and their
evolution with redshift, we use the canonical Press & Schechter (1974)
formalism. Its theoretical expression allows us to account for
the effects of inhomogeneous quintessence field through
,
the growth factor as well as the volume element. Finally, we focus on
the effects of the different models for structures with masses ranging
between 1013 and
.
This paper is organised as follows. In Sect. 2 we introduce the fundamental equations that describe the evolution of the quintessence field and the collapse of structure in the homogeneous and inhomogeneous hypothesis. In Sect. 3 we describe the method used to compute the number density of collapsed objects (mass function) in both of these scenarios, including the case where the equation of state of dark energy is allowed to vary with time. We give results and discuss the effects of the normalisation of the mass function on the predicted number counts in Sect. 4, and present concluding remarks in Sect. 5.
In a spatially flat Friedmann-Robertson-Walker Universe the cosmic
dynamics is determined by a background pressureless fluid (dark and
visible matter), radiation and dark energy. The governing equations of
motion are
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(4) |
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Figure 1:
Evolution of the dark energy density ( top panel) and equation
of state ( bottom panel) with redshift for the models
considered in this paper: w = -1 (solid line), w
= -1.2 (long dashed line), w = -0.8 (triple
dot-dashed line), 2EXP1
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Figure 1 shows the evolution of the dark energy density
and equation of state with redshift for the models we consider in this
paper.
We focus on dark energy models for which w =
-0.8, w = -1.2 (phantom energy, Caldwell 1999) and
two cases where the dark energy results from a slowly evolving scalar
field in a potential with two exponential terms (2EXP)
(Barreiro et al. 2000)
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(6) |
In this work we use the spherical collapse model to describe the
gravitational collapse of an overdense region of radius r and
density contrast
such that
,
where
and
are the energy densities of pressureless matter in the cluster and
in the background, respectively. We have considered the possibility
that dark energy also clusters, i.e.
.
Following Mota & van de Bruck (2004) and Nunes & Mota
(2004), we study two extreme limits for the evolution of dark energy
in the overdensity region. First, we assume that dark energy is
"homogeneous'', i.e. the value of
inside the
overdensity is the same as in the background. Second, dark energy is
"inhomogeneous'' and collapses with dark matter. In general terms,
the evolution of dark energy inside a cluster can be written as in Mota &
van de Bruck (2004)
In hierarchical models, cosmic structures form from the gravitational amplification of small initial density perturbations. The time evolution of the structure abundances is determined mainly by the rate at which the perturbations grow until they reach the collapse, or virialization.
An analytical computation, proposed by Press & Schechter (1974), gives
the comoving number density of collapsed dark matter halos of mass M in the interval dM at a given redshift of collapse, z by
The quantity
is the linear theory rms
density fluctuation in spheres of radius R containing the mass M and
is the linear growth
factor.
The smoothing scale R is often specified by the mass within the volume defined by the
window function at the present time (e.g. Peebles 1980). In our
analysis the variance of the smoothed overdensity containing a mass M is given by
For a fixed
(power spectrum normalization) the predicted
number density of dark matter halos given by the above formula
is uniquely affected by the dark energy models
through the ratio
.
The underlying
assumption of this approach is that the transfer function used in the
computation of
is that of a cosmological constant
model. This is a good approximation at cluster scales for
homogeneous dark energy models (Ma et al. 1999), which remains to
be theoretically investigated in the inhomogeneous hypothesis.
For a fixed local halo abundance, the important quantity to consider
is thus
.
We have verified that apart from w=-1.2 (phantom energy) all
homogeneous models have a ratio
below that of the
-model. This means that, at higher redshifts, models w=-0.8,
2EXP1 and 2EXP2 are expected to give larger halo densities (whereas
the phantom energy model is expected to give lower abundances) when
compared to the cosmological constant model. For inhomogeneous
dark energy we find that all models, except the w=-0.8 model, have
larger
than the cosmological constant model, and
therefore we expect at high redshift lower hallo densities for this
models (higher abundances in the case of w=-0.8) when compared to the
-model.
To obtain the same halo abundance at z=0 we scaled according to
,
where the index "
'' represents the cosmological constant
model, and we fix
.
Table 1 lists
the values of
obtained in this way. Note the much larger
dispersion of
between models in the inhomogeneous case
(third column) as compared to homogeneous dark energy (second column)
where models require practically the same
to reproduce the
present-day halo abundance of the
-model.
Table 1:
Variance of the overdensity field smoothed on
used in the normalization of number counts
to the present-day halo abundance in
-model
with
for homogeneous (second column) and
inhomogeneous (third column) dark energy models.
In Fig. 2 we plot the redshift evolution of the mass
function of objects with mass
for both
homogeneous (top panel) and inhomogeneous (bottom panel) dark energy
using Eq. (12).
As discussed in the previous paragraphs the w=-0.8, 2EXP1 and
2EXP2 models give larger halo abundances than the w=-1model, whereas w=-1.2 gives the lowest densities. We
find this same qualitative behaviour within the mass range of interest
for this paper,
,
for both homogeneous
and inhomogeneous dark energy models.
The counts were normalized so that at z=0 all models give the same
halo abundance as the cosmological constant (w=-1) model gives for
.
Note that the overdensity contrast at collapse,
,
is different for different dark energy models (see
Fig. 2 in Nunes & Mota 2004) and therefore the number density of
halos at z=0 is different if we assume the same
for all
models. The embedded panels in Fig. 2 show this situation
(zoomed near z=0), where
was set equal to 0.9 for all
models. Larger differences are found in the case of inhomogeneous dark
energy, because this is where
presents larger
deviations from its value in a
model (see Nunes & Mota
2004).
In what follows we assume the halo number densities of models
normalized to the present-day halo abundance of the
-model. In Sect. 4.3 we discuss the effects of
this assumption on our results.
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Figure 2:
Redshift evolution of the modified Press-Schechter mass
function at
![]() ![]() ![]() ![]() |
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In this paper we investigate the modifications caused by a dark energy
component on the number of dark matter halos. We test for one model
with a cosmological constant (w=-1) and for four quintessence
models (defined in Sect. 2). The computations are done in
the case where dark energy is homogeneous and in the case where it may
cluster, i.e. inhomogeneous dark energy.
Moreover, we choose to explore the effects on the integrated number of
dark matter halos in mass bins [
]
illustrating different classes of cosmological structures, namely
1013-1014,
1014-1015, and
1015-1016 in units
of
.
We study the effect of dark energy on the number of dark matter halos
by computing two quantities. The first is the all sky number of halos
per unit of redshift, in the mass bin
![]() |
Figure 3: Evolution of the volume element with redshift in both homogeneous ( top panel) and inhomogeneous ( bottom panel) dark energy scenarios. Lines are the same as for Fig. 1. |
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The second quantity we compute is the all sky integrated number counts
above a given mass threshold,
,
and up to redshift z:
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Figure 4:
Evolution of number counts ( top panels) with redshift and
differences from the w=-1 (cosmological constant ![]() ![]() |
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Figure 5:
Same as Fig. 4 for objects with mass within
the range
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Figure 6:
Same as Fig. 4 for objects with mass within
the range
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Figures 4-6
show the number counts,
,
obtained from
Eq. (15) (top panels) together with the difference of
counts of the various dark energy models to the
-model,
,
(bottom panels). First let us concentrate on the
homogeneous dark energy case, i.e. on the left panels of these
figures. Below redshift unity the volume element has the most
important role in the integral of Eq. (15), as it increases
by orders of magnitude. Above this redshift the volume element does
not vary much and it is the mass function that decreases by orders of
magnitude. It decreases faster for models of larger
.
Therefore, for our particular models, we expect to
see at low redshifts, in decreasing order of number of counts per
redshift, the following sequence of models: w =-1.2, w=-1,
,
and w = -0.8 and the reverse order at
high redshifts. In practice this is only true if the maximum of counts
occurs at a redshift around or greater than unity, i.e. after the
volume element does not vary much. We can verify that this requirement
is only satisfied in the lowest mass bin
,
hence the low redshift order might differ from the one
stated in the two largest mass bins. Nonetheless, the high redshift
description is accurate for all mass bins. More generally, the
differences between counts can be understood in terms of the relative
contribution to
of the integrand in
Eq. (15). The counts thus depend not only on the volume
element and on the growth factor but also on the small differences in
.
At high redshift, models with smaller
ratios imply larger halo abundances and this effect
dominates that of the volume element. At low redshifts the
differences between counts from one model to another depend also on
the masses of interest. As a matter of fact, the number of low mass
structures is mostly sensitive to the volume element whereas for
massive structures the number counts become sensitive to the
normalisation
.
The left lower panels of the
figures illustrate quantitatively the relative evolution of the number
counts. For w = -1.2, the figures depict the expected excesses and
deficits of counts compared to the cosmological constant model below
and above redshift
(defined as
). Conversely, we have
deficits and excesses below and above
,
respectively, for
all the other models. It is also worth pointing out that larger mass bins
imply a larger ratio
(see Eq. (13)), which makes the
mass function to dominate at lower redshifts in Eq. (15)
and consequently
to move to smaller values as depicted in the figures.
Now, we turn to the comparison between the inhomogeneous and
homogeneous hypotheses (right versus left panels in
Figs. 4-6).
Figure 7 is helpful to understand how the differences
arise. Indeed, when compared to the homogeneous case, we verify that
the quantity
is smaller in the inhomogeneous case
for w = -0.8 and larger for all the other models.
Given this, it is clear that for w = -0.8 we see, as
expected, smaller deficits and larger excesses of counts comparatively
to the homogeneous case and for all the other models the inverse
trend. The effects of the inhomogeneous hypothesis are more evident
for high mass bins where count deficits and excesses show larger
differences between homogeneous and inhomogeneous dark energy. For
example, in Fig. 6 we see that the w=-0.8 model has
2 times larger count excess in the inhomogeneous case.
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Figure 7:
Difference of
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For each mass bin, the differences between integrated counts
result from the
combination of effects (that act on different mass ranges) discussed
in the previous section. In our work we have obviously not performed
the integration in the mass range in Eq. (16) all the way
up to infinity but only to
.
In
the cases
and
,
integrated number counts reflect mainly
the behaviour of curves in the lowest and middle mass bin,
respectively. This is because
is dominated by
the contribution of the lower bound of the mass integration range.
Hence, it is legitimate to concentrate on the lowest mass when a
qualitative description of the integrated number counts is concerned.
We have seen in the Figs. 4-6 that in the homogeneous case, the model with
w=-1.2 presents excesses with respect to the cosmological constant
at low redshift and deficits at high redshift. All the other models
give the opposite behaviour as their
is always
lower than that of the w=-1 model. Therefore, for w = -1.2 we
expect the integrated number counts to be larger than for the
cosmological constant model until the redshift
.
The
remaining models must show the opposite behaviour. The difference
between the integrated number counts of a model with respect to the
cosmological constant must decrease with redshift for
.
Eventually above a redshift
,
the integrated number
counts should become constant with redshift because, as we have noted
before, the number counts (Eq. (16)) decrease exponentially
with redshift hence contributions above
become
negligible. Note that
becomes progressively smaller for
structures with larger mass limits (
)
because a smaller
number of these objects form at higher redshifts. This simply reflects
the hierarchical nature of structure formation in models described by
Eqs. (12)-(14). Models give quite different
integrated count differences (
)
depending on the maximum
redshift of integration. For homogeneous dark energy, maximum
deviations from the
-model are generally obtained near
of the dominant class of structures, whereas for
inhomogeneous dark energy maximum deviations generally occur at much
higher redshifts,
.
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Figure 8:
Integrated number counts up to redshift z( top panels) and count differences to the w=-1 (![]() ![]() |
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Figure 9:
Same as Fig. 8 for objects with mass
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In the inhomogeneous scenario, the interpretation of the redshift
dependence of the integrated number counts is similar to that of the
homogeneous case. As we have seen, the inhomogeneous case yields
higher
excesses and lower deficits for the w = -0.8 model, hence we expect
at high redshift, a positive difference in integrated counts
compared to the cosmological constant model. This is quite visible in
the right lower panels of Figs. 8 and 9. Through a similar argument, we expect at high
redshift, a negative difference of integrated number counts with
respect to the cosmological constant for the w = -1.2model. Moreover, because there are hardly any excesses in the 2EXP
models, we must expect the difference of integrated number counts to
flatten out at
at large negative values for these two models.
We illustrate the effects of inhomogeneous dark energy on one single
mass bin. Figure 10 shows number counts in
the mass bin
and
Fig. 11 the integrated number counts above mass
.
Both figures consider inhomogeneous
dark energy models. In the left panels of these figures we are
assuming that models are normalized to the same halo abundance at
z=0 (i.e. same curves as those in the right panels of
Figs. 5 and 9), whereas right panels
assume that all models have a fixed normalisation,
.
Models in the homogeneous hypothesis have practically the same
when they are normalized to the halo abundance at z=0.
Therefore we do not expect to observe much difference in the halo
abundances from one dark energy model to another. The situation is
quite different when dark energy is inhomogeneous. The comparison
between panels in Fig. 10 indicates that
fixing
in all models causes much larger departures from the
-model than in the case where models are normalized to
reproduce the same local halo abundances.
At the maximum of
,
the differences between dark
energy models come from the fact that for a fixed
,
the mass
function reflects the variations of
with z, which
effects dominate those of the volume element in Eq. (15)
for this mass bin. It is interesting to note that the structure of the curves can change dramatically depending weather we fix the local abundance or
.
As this work was being completed Manera & Mota (2005) made public an
analysis similar to the one presented here for the particular scenario
of coupled quintessence. In their work, they have fixed
for
the two models under study rather than the local halo abundance. We
have verified that indeed, also in the coupled quintessence model, the
departures from a cosmological constant model are larger if one takes
the former approach.
![]() |
Figure 10:
Redshift dependence of number counts ( top panels) and count
differences relative to the ![]() ![]() ![]() |
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![]() |
Figure 11:
Integrated number counts up to redshift z ( top panels) and
integrated count differences relative to the ![]() ![]() ![]() |
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More specifically, we have considered a scalar potential built out of
two exponential terms for which two sets of parameters (models
2EXP1 and 2EXP2) were explored. Although their equations of
state at present are almost indistinguishable,
they undergo quite different evolutions at higher redshifts and
generally give different results. In our analysis we have also
included a phantom dark energy model (w=-1.2), compatible with
present observations. An interesting feature of phantom dark energy
is that it induces opposite departures from the -model as
compared with the other models considered in this paper. That is, we
expect an excess of sources in a phantom energy model when other
models predict a deficit.
Recently Mota & van de Bruck (2004) proposed that dark energy may
cluster in forming structures (inhomogeneous dark energy). They have
investigated the growth and collapse of cosmological structures under
the inhomogeneous hypothesis. Going a step
forward, we have investigated in the present study the implications of
this hypothesis on the number density of collapsed objects.
We have found that inhomogeneous dark energy generally enhances departures
from the -model. This includes the models with a time varying
equation of state, which can present several times
larger departures (from the
-model) as compared to the
homogeneous case. Yet our results indicate that the inhomogeneous
dark energy hypothesis causes maximum deviations no larger than
15% in mass bins with comfortably large numbers of collapsed halos.
Another interesting feature is that
maximum departures from the
-model are generally obtained at higher
redshift for inhomogeneous dark energy than for the homogeneous case,
which generally show maximum departures near the maximum of
.
This may be a helpful feature to test for the
inhomogeneous hypothesis.
Larger departures from the
-model are also stronger for the
more massive structures, but these are quite rare objects, which makes
it difficult to statistically distinguish between models.
Our analysis reveals that the inhomogeneous dark energy hypothesis has
the greatest impact on the 2EXP1 model.
In this work, we have assumed that the matter transfer function
remains unchanged at cluster scales. We have further assumed that
models are normalized to reproduce the same abundance of dark mater
halos at redshift zero. In the homogeneous hypothesis all models have
practically the same
and there are not much differences in
the halo abundances from one dark energy model to another. When dark
energy is inhomogeneous,
differ by a few percent and the
departures from the
-model are much larger. It is, however,
worth noting that the gas physics which rules the observed quantities
adds a degree of degeneracy. We have evaluated the effects of
alternatively fixing
to a specific value regardless of the
dark energy model. In this case we verified that the departures from a
cosmology are further enhanced.
Our results show that constraining dark energy models from structure counts is complicated when models have time varying equation of state. It becomes an even more complicated task when the possibility of inhomogeneous dark energy is taken into account. Therefore in order to constrain dark energy models, we need to explore as many observable quantities as possible. Our results suggest that besides redshift distribution of structures, considering structures in mass ranges significantly increases the number of observables. Indeed, each theoretical model provides specific predictions for the redshift evolution of number counts and integrated count differences in different mass bins. The comparison of such quantities with observations can be used for testing models against observational data. It may further allow to distinguish between homogeneous and inhomogeneous dark energy models. However, this requires good knowledge of the gas physics, redshifts of observed structures and, more precisely, a good understanding of the selection function of the observations.
Acknowledgements
We thank David Mota and Morgan LeDelliou for invaluable discussions. N.J.N. is supported by the Department of Energy under contract DE-FG02-94ER40823 at the University of Minnesota. A.d.S. acknowledges support by CMBnet EU TMR network and Fundação para a Ciência e Tecnologia under contract SFHR/BPD/20583/2004, Portugal. This work was partly supported by CNES. We would like to thank the referee for his/her comments.