Validate by checking a variable

You see, I have the table Photo, with these values:

    Schema::create('fotos', function(Blueprint $table){
        $table->increments('id');
        $table->string('foto');
        $table->string('nombre');
        $table->date('fecha');
        $table->unsignedInteger('user_id'); // Foraneo a la tabla User.
        $table->foreign('user_id')->references('id')->on('users');
        $table->timestamps();
    });

I have a form that modifies the data of the photo:

@extends('layouts.app')
@section('content')
    <div class="container">
        <div class="row justify-content-center">
            <div class="col-md-8">
                <div class="card">
                    <div class="card-header"><h1>Corregir información sobre la foto</h1></div>

                    <img class="card-img-top" src="{{url($f->ruta())}}"/>

                    <div class="card-body">
                        <form method="POST" action="{{route('foto.cambiar',$f)}}" enctype="multipart/form-data">
                            @csrf

                            <div class="form-group row">
                                <label for="nombre" class="col-md-4 col-form-label text-md-right">Nombre</label>

                                <div class="col-md-6">
                                    <input id="nombre" type="text" class="form-control{{ $errors->has('nombre') ? ' is-invalid' : '' }}" name="nombre" value="{{ $f->nombre }}" required autofocus>

                                    @if ($errors->has('nombre'))
                                        <span class="invalid-feedback">
                                            <strong>{{ $errors->first('nombre') }}</strong>
                                        </span>
                                    @endif
                                </div>
                            </div>

                            <div class="form-group row">
                                <label for="fecha" class="col-md-4 col-form-label text-md-right">Fecha en la que se creo la foto</label>

                                <div class="col-md-6">
                                    <input id="fecha" type="date" class="form-control{{ $errors->has('fecha') ? ' is-invalid' : '' }}" name="fecha" value="{{ $f->fecha }}" required autofocus>

                                    @if ($errors->has('fecha'))
                                        <span class="invalid-feedback">
                                            <strong>{{ $errors->first('fecha') }}</strong>
                                        </span>
                                    @endif
                                </div>
                            </div>

                            <div class="form-group row mb-0">
                                <div class="col-md-6 offset-md-4">
                                    <button type="submit" class="btn btn-primary">
                                        Subir Foto
                                    </button>
                                </div>
                            </div>
                        </form>
                    </div>
                </div>
            </div>
        </div>
    </div>
@endsection

And it goes through this validator:

public function rules(){
    return[
        'nombre'=>'required',
        'fecha'=>'required|before_or_equal:'.date('Y-m-d')
    ];
}

This checks that the date does not exceed the current one, but I want instead that the date does not exceed the value of created_at . How do I do that?